Hollow Moon Planetology: Blue Moon
by 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:
http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/lo4_m187.html
(** - 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. ;-)
Crystal passages
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.