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Hollow Moon Planetology: Blue Moon - Crystalbarrensby Sharon Dornhoff
By far the most unique features of Hollow Moon geography, "the crystalbarrens" are what Materans call those parts of Maria Serenitatis, Imbrium and Nubium, and of the great Oceanus Procellarum, which are not a part of the Midland Ocean or its surrounding ice-flats. They are the HM setting's answer to both desert and tundra: cold, dry, wind-swept expanses of bare, level crystal, dotted with islands of sand-crust or basalt cones, and lit from beneath by a deep blue light, in the darker portion of the lunar "day". Intolerable to most forms of humanoid life, for their frigid temperatures and sombre desolation, they are nevertheless home to two of Matera's hardiest and most ancient races, the cryion bat-people and the elemental desert ghosts. The former are nomads who cluster upon the islands and ice-flats, where conditions are marginally more favourable and food can be herded or grown; the latter, being dust-spirits who require neither food or warmth, are perfectly comfortable on the open, arid wastes, and indeed they avoid the humidity of warmer, elevated locales.
Conditions on the open crystalbarrens are arctic, windy and arid; they also offer a few twists that conventional polar lands on Mystara don't exhibit. Because the "ground" of the inner maria is actually contiguous with the maria of the outer moon, the translucent bedrock literally sucks the heat out of any unprotected flesh that comes in contact with it, inflicting 1 hp damage per turn upon travellers who lack protective footwear. Fortunately, this "heat-sink" effect is only felt by immediate contact with the crystal, and does not apply on the various "islands". Temperatures are a steady -30 degrees F at ground level -- the same as the outer moon's surface beneath 3' of depth -- but increase by a whopping 5 degrees per foot of elevation, as the wider, volcano-heated atmosphere of the Hollow Moon supplants the chill that clings to the crystalline surface. (Don't expect it to get too comfortable, though; the mean air temperature for the HM setting, as a whole, is only 40 degrees F, so even the highest points on the islands are only a few degrees above freezing.) The cryions' wind-powered ice-sledges, by which they traverse the open wastes, are built with high prows and elevated decks, the better to keep their crews clear of the worst of the cold. Given the odd fact that it's the ground, not the wind, which is most likely to freeze you to death, out on the crystalbarrens -- the opposite of what it's like in ordinary polar regions -- the Survival (arctic) skill is of very little use in these regions; the special Survival (crystalbarrens) skill is required to find food, water or shelter there.
In the week-long period after fulldark, when greenish direct sunlight shines up from the ground, temperatures climb even higher in the equatorial region of the Oceanus Procellarum, increasing by 10 degrees per foot of elevation. It's at this time -- when above-freezing temperatures lie just 8' above the crystal -- that the cryions' few frost-tolerant crops can sprout and set seed on the islands, before the sun passes beneath Procellarum's western shoreline and the chill settles in again. The appearance of the green light underfoot, and associated change in temperature, also generates a thick, ground-hugging mist that condenses on the crops, islands and cryion villages, providing the mnelds with a monthly ration of precious moisture in an environment where springs and rivers are unknown. The few mosses and lichens that grow below the 8' "freeze-line" on Oceanus Procellarum's islands, or in non-equatorial zones of the crystalbarrens, must wait until storms inundate their islands and briefly thaw out the soil ... an event as uncommon as, and having similar ecological importance to, the monsoons which irrigate the sere nation of Sind on Mystara. Hardy musk-oxen and caribou browse on the lichen, and are herded by cryion drovers.
Winds on the crystalbarrens are powerful and unceasing, and they don't conform to the usual circulation patterns for a planet (Matera's rotation is too slow to generate a significant corriolis effect). Instead, warm air from the various volcanic lands surrounding the Oceanus Procellarum blows into the crystalbarrens at high altitudes, and ground-level cold winds blow out onto the land. The periodic events called "circle tides" interrupt this pattern, causing winds to alternately blow from south to north, or from north to south, each time they occur. On Maria Imbrium and Serenitatis, the extreme cold of the Mare Frigoris region and the humid warmth of the Midland Ocean create a weather front that shifts back and forth, occasionally dropping rain on the crystalbarrens, to the relief of the Caucasus Modrigswerg and the troglodytes of Palus Putredinis ... and to the abject horror of the desert ghosts, to whom liquid water is deadly. (Luckily for the Hollow Moon's oldest inhabitants, that "toxic" rainfall usually turns to snow before hitting the ground, and is blown away harmlessly by the prevailing winds.)
On the fringes of the Vesper Ocean, where the warmth of the midlands peters out and the waters become icy, lie some 30 miles of ice-flats, which have built up atop the open crystal over the millennia. While the central ocean's pressure and submarine vulcanism can keep its nearward regions from freezing onto the crystal bedrock, the outlying waters closest to the edge of the crystalbarrens are perpetually solidifying upon the chill, translucent seabed (-30 F freezes seawater easily), only to crack apart, sending sheets of ice bobbing to the surface. These ice-sheets, interspersed with horizontal pockets of seawater, build up month after month along the ocean's edge, as layers break free of the frigid bottom. As depth decreases nearer to the shore, the layers of ice-sheets grow thicker and thicker, until eventually the ice becomes solid clear down to the seabed, forming an open plain that never thaws. Seasonal spring tides wash over the ice twice a year, and have added to and "smoothed over" this borderline. Daring cryion sailors sometimes pilot their sledges directly onto the ice, as the transition from crystal to ice-flats is smooth and gradual. Level at their outermost edge, the ice-flats become broken and treacherous as one approaches the sea, and the solid ice gives way to ice-covered ocean. Seals and other arctic mammals prosper along the ice/sea fringe, as the cold, oxygen-rich waters that underlie the ice-flats foster an abundance of fish; algae to support these fish is as likely to grow near the bottom, using light that shines up from the underlying crystal during fulldark, as near the surface where the Firmament's glow can reach them, the rest of the month.
As for the crystal bedrock itself, it -- as already mentioned -- is completely impervious to mortal weapons, spells, or feats of engineering. Magical effects such as Stone Shape or shadow elven tunnel-moving magics won't alter its shape, although Glassee will render its colourless translucence even more transparent. When sunlight shines upward through the crystalbarrens, they take on much the same indigo glow as does the Firmament (which they should -- they're the same material). When they aren't lit from below, the crystalbarrens appear quite dark, offering a stark contrast to the white of the ice-flats, to the pale mists that enshroud the equatorial regions once a month, or to the white-furred cryions themselves. Although dust and rock fragments may make the surface appear coarse, the crystalbarrens are actually quite smooth and slick; walking on these lands' surface feels as slippery as walking on ice, except where dust or grit provide traction, and sledges, skates, skis, and cryions' specialised skating-claws are all able to glide across this terrain as easily as they can upon level ice.