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Hollow Moon Planetology: Blue Moon - Lunar Geographyby Sharon Dornhoff
((Everyone found a moon-map by now...? If not, go get one -- otherwise, most of this won't make any sense! Any map of Luna's visible surface that includes the major craters' names will do; just remember that the Hollow Moon is a mirror image of what you're looking at.))
Compass directions: mapping the Hollow Moon
Directions in the Hollow Moon setting can be tricky. North, South, East and West can all be used; but they don't always match up, when using different IRL moon-maps. Older maps of Luna often put South on the top, and the definition of "east/west" can vary depending on whether the mappers are taking an "Earth's-eye" perspective, or a "Moon's-eye" one. In addition, there are two other directions -- "nearward" and "farward" -- which visitors to the Hollow Moon had better get used to: Native Materans use them all the time.
The North and South Poles of Matera mark the two ends of the axis upon which the moon rotates; the midpoint between them is the lunar equator. Because the moon's axis of rotation is not as tilted as Mystara's (less than 2 degrees, as opposed to 23), the Materan equator always receives its sunlight head-on. The Northern and Southern Hemispheres of Matera -- like those of Mystara or Earth -- encompass everything from their respective poles to the equator; most of the Nearside's maria are in the Northern Hemisphere, while Mare Orientale is in the Southern Hemisphere. The "north" and "south" directions are understood and used freely by the Hollow Moon's inhabitants, and shouldn't cause a problem.
East and West are where things start getting confusing. To space-explorers approaching from Mystara or observers using a telescope, the half of the moon which looks like "West" is the side which includes Oceanus Procellarum. (This is the biggest dark area you see, when you look up at Luna IRL.) The other half, with several smaller dark areas, is usually called "East" by Mystaran astronomers. That's how the PCs' maps of the outer Matera that they bring from home will probably be labelled, and how most IRL moon-maps label their compass-points as well.
Unfortunately, once PCs have actually arrived in the Hollow Moon, they'll encounter a big problem with this perspective: it's over the Oceanus Procellarum, where the lunar "dawn" occurs! The brightening of the Firmament after fulldark, and the actual appearance of the sun later in the month, take place in what a Mystaran map calls the "west"... the exact reverse of the customary situation for a planet*; and a reversal that's likely to get the first PC who becomes lost in the Hollow Moon moving in the wrong direction! Likewise, the lunar "dusk" actually transpires on what a Mystaran map would call the eastern edge of the Nearside, as the sun sinks beneath Mare Foecunditatis -- backwards, again.
[* - In the Hollow World, of course, there's no sunrise OR sunset, only noon; so that setting's reversed east/west directions can't become an issue, the way they can in the HM setting.]
Among native Materans, the corresponding two directions are "dawnward" and "duskward": the former means the astronomer's "west", and the latter, the astronomer's "east". Unfortunately, attempts to translate these terms into Common are likely to give the opposite answers, due to translators' preconceptions about where the sun rises and sets! This could confuse the heck out of explorers -- all the more so, if they don't realise the Hollow Moon has a recognisable dawn or dusk, at all. (For purposes of these posts, I'll be using "west" to refer to the direction of the lunar sunrise and "east" for that of the lunar sunset, in accordance with the moon-maps DMs are likely to be using.) The terms "Eastern Hemisphere" and "Western Hemisphere" aren't used in the Hollow Moon.
Along with north, south, dawnward, and duskward, natives of Matera also have words to describe orientation relative to the centre-point of the Nearside. "Nearward" and "farward" respectively indicate a direction toward, and away from, the lunar point which comes closest to the planet Mystara. (While Materans don't actually know that Mystara is OUT there, they know that their whole world is roughly-spherical, and that the habitable half of it -- the part that's not Firmament -- has a midpoint.) On moon-maps IRL, lunar coordinates (0,0) mark the exact centre of the Nearside; this can also be thought of as a "pole" for an imaginary "Nearward Hemisphere". Motion toward this geographic centre-point is "nearward" motion. Conversely, lunar coordinates (180,0) mark the centre point of the Farside -- i.e. the "pole" of a hypothetical "Farward Hemisphere" -- and moving toward this distant point (and away from (0,0)) constitutes "farward" movement. On a flat, circular map of just the Nearside, "nearward" means toward the middle, and "farward" means toward the edges; on a similar map of the Farside/Firmament, the reverse is true.
Lands along the edges of the Nearside are called the "rimlands", while those in the near vicinity of coordinates (0,0) are called the "midlands". (If I've lost you, try thinking of Matera's habitable hemisphere as a bowl that's balanced on the end of a long pole: the pole represents a line from Mystara to Matera; the bowl's rim is the rimlands; and the place the bowl touches the pole is geographic point (0,0), at the centre of the midlands.) The special terms "rimward" and "midward" are used only on those peninsulas of habitable land that jut out beyond the Nearside/Farside border (located at coordinates 90 E & 90 W longitude) -- compared to which all the other Materan realms are "nearward" -- to better distinguish moving toward regions on the fringe of the Nearside from moving all the way to the midlands.
Matera, like Luna, has no magnetic field. Non-magical compasses won't work there, although magic ones will. Native mages of Matera have direction-finding spells that distinguish both the north/south directions, and the nearward/farward ones.
Geographic overview: how the inner and outer moons correspond
Apart from being mirror-images, the geography of the Hollow Moon and of the outer Matera are identical in their layout, sharing virtually all of the same maria, highlands, and geology. Not only are the open stretches of crystal which can be seen from space also exposed within the satellite's interior -- albeit sometimes covered by seawater, freshwater, and/or ice -- but the interior landscape of the lunar highlands also corresponds to that of the outside ... so much so, that a good crater-map of Luna can be the perfect guide to the mountains and island chains within Matera! (In fact, I'll be using the Earth-names of craters as landmarks, from here on in. Native Materans have their own names for them, and Mystaran astronomers surely do as well; but I'm not going to start re-naming geographic features until we've worked out a cultural motif for each of the major lunar societies. :-D)
Of the several outer-moon "seas", ten of them -- Maria Crisium, Foecunditatis, Frigoris, Humorum, Imbrium, Nectaris, Nubium, Serenitatis, Tranquillitatis, and Vaporum -- plus Oceanus Procellarum, are also represented by exposed crystal in the Hollow Moon. Some of the "bays", "lakes", and "marshes" which adjoin these "seas" -- Sini Aestuum, Iridum, Medii and Roris; Lacus Somniorum; Pali Somni and Putredinis -- may also be represented in the inner world, depending upon their proximity to other regions of exposed crystal, the adjacent terrain, and whether their IRL names inspire the imagination (What DM could pass up the chance to take a name like "Marsh of Putrescence" literally...? ;-D).
Of the moon's ten crystalline-bottomed interior maria, four -- Foecunditatis, Tranquillitatis, Crisium and Humorum -- are saltwater seas; Foecunditatis and Tranquillitatis are connected to one another by a narrow strait, while Crisium and Humorum are bounded by land and have no direct connection to other maria. Another body of water -- known to various native cultures as the "Midland Ocean", "Vesper Ocean", or "Great Spindrift Sea" -- encompasses the Mare Vaporum, Sinus Aestuum, and Sinus Medii ... plus ALL parts of Maria Imbrium, Nubium, and Serenitatis or the Oceanus Procellarum which lie within 25 degrees of the Nearside's midpoint at lunar coordinates (0,0)*. Another 30 miles of ice-flats surround this Midland Ocean wherever it peters out on open crystal, as these regions are periodically flooded by the Hollow Moon's tides, then freeze solid between inundations. The remainder of Maria Imbrium, Nubium and Serenitatis, like the better part of Oceanus Procellarum, are exposed, frigid, desolate crystalbarrens. Mare Nectaris -- true to its name -- is a freshwater lake: the only body of fresh water in the Hollow Moon to sport a crystalline bottom. Mare Frigoris, and the adjacent Sinus Roris, are permanently ice-sheathed.
[* - Don't bother worrying about how many miles this is; just get out the ol' compass and draw a big circle on the "maria" portions of your moon-map, using 25 N & S latitude and 25 E & W longitude as guides.]
The Hollow Moon is EXTREMELY active geologically, with more live volcanoes per square mile than can be found in any part of Mystara, barring uninhabitable deathtraps like Sind's Burning Waste. In fact, vulcanism is responsible for every mountain, sinkhole, island or caldera within the HM setting! Unlike the volcanoes on most worlds, those within Matera aren't a product of plate tectonics or vortices to the Plane of Fire; rather, the many "hot spots" which generated the inner moon's rugged terrain are a direct product of asteroidal or cometary impacts on the satellite's exterior. Billions of years ago, during the Hollow Moon's geological development, such impacts transferred sufficient energy into the crystalline bedrock of Matera to superheat rock-layers beyond the heat-resistant crystal, producing interior volcanic cones of all sizes. Once a "hot spot" is generated in the interior of Matera, it rarely cools down entirely; tidal influences from the fast-orbiting, super-dense Patera exert sufficient gravitational stress to keep the Hollow Moon's depths molten and active, even millions of years after the most recent impact (the one that created Mare Orientale) struck the visible moon.
Over time, many of the older or broader volcanic cones -- Deslandres, Ptolemaeus, Fracastorius, Hipparchus, Clavius, Janssen and others* -- have collapsed, forming calderas, lakes, badlands, or plains of rich volcanic soil. Other inland peaks*, such as Arzachel, Langrenus, Aristoteles or the mighty Mt. Copernicus, grew to fantastic heights in the low-pressure airlessness that prevailed within Matera, before the Immortal Ka and his associates thickened and oxygenated its atmosphere. Actual "ranges" of crater-born peaks are all but unknown, in the Hollow Moon, as the basalt-heating impacts which created them tended to occur at widely-separated points in geologic history; the chief exception is the sprawling Stofler-Walter-Purbach-Arzachel Range that overlooks Mare Nubium. This long stretch of rugged semi-active peaks resembles parts of the U.S. Pacific Northwest, and its mountains date back to a series of impacts by a single asteroid that split into pieces just before collision, tracing a line of "hot spots" not unlike the comet that struck Jupiter a few years ago IRL. Other so-called mountain "ranges", such as the Taurus and Caucasus, are actually lone calderas which fragmented into many smaller cones, that later grew independently. They don't form straight lines of peaks, the way mountains tend to be arranged on planets with normal tectonic activity.
[* - Get out those moon-maps; all these are IRL craters you should be able to find on them! All craters on the outer Matera or Luna correspond to either mountains, or collapsed calderas, in the Hollow Moon. Pretty handy, huh? :-)]
While mountains in the "inland" parts of the moon are numerous and densely-packed, volcanoes in the maria are few and far-between. Impacts that hit bare crystal, on the outer moon, didn't have any solid basalt to heat up, on the other side of the Materan bedrock. However, some of them did transfer sufficient residual heat to the moon's arid interior, to fuse encrustations of dust upon the inner surface, creating "islands" of metamorphic rock within the crystalline desolation of various lunar maria. Some of these islands, such as the Fra Mauro or Haemus Isles, are large enough to sustain their own vulcanism, and habitable to all manner of life because of geothermal heat. Others, like the Carpathian or Riphaeus-Ural Archipelagos, are too flat and ground-hugging to retain active magma beneath their surface, offering "oases" of inhabitable land in the midst of the crystalbarrens only for races which -- like the cryions -- are capable of living comfortably in a sub-zero environment. Encrustation-flats in Mare Vaporum and Sinus Medii have long since been inundated by seawater ... and the maeshimer of the Great Spindrift Sea have worked many architectural wonders, with these stretches of fused-dust seabed.
Not all of the terrain within the Hollow Moon is a product of individual asteroid-strikes on Mystara' satellite. By far the oldest features on the Nearside, apart from the maria themselves, are a handful of bluffs and ancient mountain ranges that originated with the marias' creation (more about this later). The loftiest of these are the Apennines, which mark the southeastern corner of Mare Imbrium. Originally several times higher than Mt. Copernicus, they're still the steepest of the Hollow Moon's few mountain-clusters, despite the fact they've been shedding magma into Mare Vaporum for the last billion-odd years. The Alps and Jura Mountains, also surrounding Mare Imbrium, are much less impressive, although equally ancient; the Juras were volcanically "dead" for most of Matera's existence, but have been "revived" by nearby cometary impacts in the relatively-recent geological past. Running roughly parallel to the shoreline of Mare Nectaris, the Altai Scarp rises up in an immense plateau; its cave-pocked surface was eroded by windblown moon-dust, long before the Hollow Moon was made habitable.
The last catastrophic event to shape the Hollow Moon was the impact which created Mare Orientale, raised the Rook and Cordillera Mountains, and opened the interior of Matera to a wider universe for the first time. This same impact also produced visible fracture-patterns thousands of miles long, within the crystalline structure of the Firmament; split open a three-thousand-foot fissure, Venturer Deep, in the silted seabed of Mare Humorum; and gouged out a eighty-five mile crevice, the Alpine Valley, when the ricocheting core of the comet that'd penetrated Matera finally came to ground in the northern mountains. More recently (geologically speaking), the introduction of water and a breathable atmosphere by the Hollow Moon's Immortal protectors have generated such surface features as "the Rilles" -- an Everglades-like region of marshes and hummocks -- on Mare Vaporum's coastline, or the island-dotted Lake Clavius in the south. All but a few regions of desolate land, such as the arid wastelands of Sohktar or the arctic Shadow Zone near the South Pole, have since become fertile, vegetated, and populated by animals and lost races.
So what's this mean for my PCs...?
From an adventurer's point of view, the fact that vulcanism is responsible for virtually all the Hollow Moon's landscape may not seem very important. However, one consequence of this geologic history is very relevant to a would-be spelunker: There are NO limestone caves, of the sort adventurers are accustomed to exploring, in the Hollow Moon! (In fact there's no limestone, period; there hasn't been water in Matera long enough for such sedimentary calcite to form.) Likewise, there's no familiar stalactites and stalagmites; gargantuan Underdark-style caverns, of the sort common to Graakhalia, the Broken Lands, and shadow elf territory; or -- since the basalt's far too tough for mere running water to erode through -- navigable subterranean rivers. Instead, cave-delvers should be prepared for meandering lava tubes, zigzag fissures along volcanic fault lines, and toxic or explosive gas-pockets. Many underground areas have magma very close to them and are sweltering-hot; others contain pools of boiling mud or geysers that can spray the unwary, or fill up periodically with steam from superheated springs. The few natural caves in Matera which aren't volcanic in origin are the products of erosion by wind or wave action ... like the Altai Scarp's shallow, phanatrel-populated overhangs; or the smooth-walled sandstone caverns along the southern coast of Mare Crisium.