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Hollow Moon Planetology: Blue Moonby Sharon Dornhoff
((Hang on for the ride, folks -- here's the start of what'll probably become THE longest thread we've seen on the Mystara board, for a while: All the facts on the Hollow Moon interior environment! But first....))
Prologue: How to get in
Once your PCs have successfully sailed -- or flown, teleported, hopped there on giant moon bunnies, whatever! ;-) -- to Matera, finding their way into the Hollow Moon is actually quite easy. There's only one permanent access-point into the lunar inner world, but it's a doozey: the gargantuan impact crater at Mare Orientale, which is plainly visible to anyone orbiting Matera or approaching the Nearside/Farside border from the "lunar" west (i.e. the side with the largest "sea", Oceanus Procellarum). Not only is the crater, itself, enormous -- roughly 170 miles across -- but it's surrounded by two concentric rings of mountains, the Cordilleras and Rooks, the loftiest peaks of which reach 5 miles up from the Farside's crystalline bedrock. All told, this immense "bulls-eye" pattern surrounding the Mare* is nearly 700 miles in diameter ... this, on a moon that's only 2160 miles from pole to pole. Mare Orientale doesn't punch directly into the "golden dome" of the Farside's exposed crystal, on Matera. Instead, the bull's eye and tunnel into the Hollow Moon are located on the largest of several peninsulas of ordinary moon-rock which extend out onto the Farside surface, jutting well beyond 90 degrees** longitude, from the Nearward Hemisphere.
(* - For some neat views of Mare Orientale -- without a doubt, the most spectacular IRL impact crater in the known solar system! -- check out this website:
(** - In lunar geography, the boundary between the near and far hemispheres is at 90 degrees longitude, both East and West; and (0,0) are the coordinates for the exact centre of the near side, which is the nearest lunar point to Earth/Mystara. Mare Orientale can be found at longitude 94 W, latitude 12 S.)
While the impact crater is huge at the surface, it's not so wide all the way down. Once the PCs have entered Mare Orientale, and dropped below the 12 miles of basalt, fragmented crystal, and ancient comet-debris which comprise the Mare's peninsula, they'll find the passage shrinks to a diameter of only 4-7 miles. From that point, it also ceases to run straight down, but veers and meanders and splits off dead-end side passages, like an irregular crack running through a flawed gemstone. At the DM's discretion, the robotic Blackmoor probe which once got lost in these twists and turns could be found here, intact; it is also a good place for Immortals to imprison otherplanar beings they can't or won't destroy, by walling them up inside one of the crater's numerous cul-de-sacs behind screens of thin, but unbreakable crystal.
These cul-de-sacs make such good prisons, even for ethereal creatures or super-powerful fiends, by reason of their material. At the same place where the passage narrows, basalt and encrusted space-dust give way to the same impenetrable, translucent, crystalline bedrock which makes up the Nearside Maria, and is exposed on 82% of the Farside's surface. It can't be broken, moved through by an incorporeal being, or teleported through. This dense, quartz-like bedrock -- which only the stupendous cometary collision* that created Matera's Mare Orientale had the force to perforate -- is 30 miles thick; and there's an abrupt gravity inversion inside it, at 15 miles' depth ... the halfway-point between Matera's outer and inner surfaces. As with the gravity-plane that spans the vast cavern of the shadow elves' City of Stars, this sudden flip-flop of gravity could easily catch PC explorers off-guard, having the same effect upon voidships as a Reverse Gravity spell. (That's assuming the SJ rules aren't in effect. If Spelljammer-style gravity applies, dropping through the gravity-plane won't affect a ship at all.)
(* - "Armageddon" and "Deep Impact" had nothing on this one!)
Assuming the PCs' vessel survives the gravity inversion, they'll quickly discover that on the other side -- not confined by any discernible barrier, but simply reaching the gravity-plane from within the moon, and then stopping -- is a breathable, albeit very thin, atmosphere. This rarefied air can be collected in sacs or used to refresh an SJ air envelope for a return voyage; it won't, however, pass through the gravity-plane on its own, even if a Gust Of Wind is used to direct it at the other side of the inversion. Air that is dragged across in containers, and is allowed to leak out of them at any point within the 15 miles of crystal tunnel which are subject to outer-moon gravity, will actually "pour" out of its containers like a liquid, "spilling" down to rejoin the inner atmosphere! This curious phenomenon retains the Hollow Moon's atmosphere, but it doesn't inhibit creatures (even gaseous ones) from moving across the gravity-plane. PCs who fall overboard in the crater, in fact, will find themselves gradually bobbing to a halt in the plane of the gravity-inversion, one side of their bodies in air and the other in "heardium" Void-gas.
Temperatures within the Mare Orientale access tunnel are a nippy -30 degrees F ... warmer than the shadowed side of the outer moon, yet still far from comfortable. Because the crystal is covered in 12 miles of rock, no sunlight can penetrate this far, so the journey through the tunnel is made in complete darkness. The exposed surfaces of the crystalline walls are highly polished and smooth, as if they were once melted by incredible heat; they are also highly reflective, unlike similar crystal of the inner or outer Matera, and any lights the PCs bring with them will cause the tunnel walls to gleam like fine glassware.
After another 15 miles of switchbacks and blind alleys -- which, along with the ones encountered on the outer-moon side of the gravity-plane, may triple or quadruple the actual distance the PCs must cover -- comes another 9 miles of volcanic rock, broken fragments of crystals, and debris, along with a gradual widening of the passageway to a diameter of about 30 miles. At the far end of the tunnel, the opening expands to a gaping maw 120 miles across. Double rings of mountains -- the Hollow Moon's own 5-mile-high Cordilleras and Rooks -- encircle the crater's mouth, on their own Farside-jutting peninsula, hiding much of what lies beyond from immediate view. What they see, when they rise above the peaks, will be revealed in post 3a. ... coming up soon. ;-)
While Mare Orientale is the most obvious way to cross over into the Hollow Moon, it's not the only option. Visitors who set out to learn everything they can about Matera will eventually find out -- either through independent spell research, or through contact with descendants of the Nephthisian Exodus -- that certain powerful spells can create temporary openings through the crystalline bedrock of the Maria or the Farside, can allow a space-traveller to phase through the otherwise-impenetrable crystal, or can even seal off an existing passage for a short time (as the Nithian fleet found out the hard way).
While sealing off Mare Orientale, itself, would probably require Quest-level magic -- which is what the priestesses of Nephthisi used, to save themselves -- smaller openings can be created as needed in the Farside's "golden dome"; or in the bedrock of any of the Nearside Maria, with the exception of the few that aren't crystalline. No matter where a crystal passage is created, the bedrock it penetrates is always 30 miles thick, with a similar gravity-plane to the one in Mare Orientale, at its midpoint. Unlike the impact crater's passage, such magically-created passages through exposed crystal are straight, and of a constant diameter. Their walls are smooth, but unpolished and non-reflective. If sunlight is falling on the side of Matera which these passages penetrate, the brilliant, burning light of the outer moon will shine into their depths, as far as the angle of incidence of that light allows. Crystal passages are short-lived phenomena and collapse quite rapidly when their duration expires, potentially crushing travellers who aren't able to reach one exit or the other in time. (Note that CoM Voidships move at their atmospheric speed, in the air-filled portion of a crystal passage.) Air in the passage gusts out into the Hollow Moon, as it collapses; and a very lucky flying creature who's trapped in that half of the tunnel might get blown out into the inner moon by this wind, instead of crushed. It will still take some damage from collisions with the walls, however: it's a bumpy ride. ;-D
A few of the Maria on the Nearside have shallow seas on their opposite sides, rather than air. If a crystal passage is opened into the floor of such a sea, the waters of the Hollow Moon will rush in, but it will behave much as Hollow Moon air does and not cross the gravity-plane; when the passage's duration has expired, the water is gently squeezed back up into the sea. Unlike a collapsing passage that's only filled with air, a water-filled passage closes slowly enough so that beings in the water aren't harmed, as they get "squirted" back into the lunar seas. On the other hand, the effect of sucking so much water into the passage in the first place is damaging to sea life and marine races, in itself; this careless practice won't win the PCs any friends, in the HM setting.
The fringes of certain Maria, and of Oceanus Procellarum, are covered in heavy glacial ice. Opening a crystal passage there, unfortunately, won't do a space-traveller much good, as this ice is often up to a half-mile thick! Likewise, even though it's possible to follow a cave or crevice on the exterior Nearside all the way down to bedrock, it'd take an unbelievable coincidence for similar caves to exist, on the other side of the crystal: when opened, most such passages lead only to solid stone or -- for the truly unfortunate -- molten lava. Even if the long odds pay off, and a spell-opened subterranean passage leads through the crystal to more tunnels underground, there's absolutely no guarantee they'll be open to the interior, and not dead ends. (Such cul-de-sacs invariably lack air inside them, so PCs should at least realise they're going nowhere, when they reach the 15-mile mark and find no atmosphere.) These blind cavern-networks are the product of the Hollow Moon's intense volcanism, and provide another handy place for Immortals -- or perhaps, high-level NPC mages -- to hide their secrets, stockpiles, treasures, private laboratories or imprisoned enemies.