Cave

Its a series of tubes. No, really. 6 miles – 10 km east from HAB - Site Epsilon Grey rock Lava tubes covered by quarts:
 * Location
 * Description

“It’s… it’s incredible,” Starlight gasped. “It’s like the caverns under Canterlot!”

"''Greetings from the Fortress of Solitude!" --Mark Watney''

'' There were even a couple of shafts of crystal that looked as thick as I am that went from ceiling through the floor.''

about 600m deep in total 

''Anyone trying to imagine the cave should read the sol 20, sol 21, sol 204 and sol 330 chapter. ''


 * The entrance: 80 meters of "Really big" tunnel.

Contains one of the Friendship airlocks (Sol 44)

''Crystals around airlock are all removed prior to putting it in place. sixteen hooves of no crystals''

Some of these crystals where left just outside

In fact not the start of the cave. The cave continues in the other direction. This was originally a skylight,

This other end of the cave has not yet been explored, but was included on the sol 204 hologram. Several meters high at most, still, By far the tallest place in the cave.
 * The Farm(Cave) 20m X 200m 

''a couple of crystal shafts that run from ceiling to floor here and there. ''

''21 meters wide at widest point.

Broken pillars line the walls

Contains the Stump:

It was a rough-broken giant end of a quartz crystal at first, sticking out not quite a meter from the surface of the cave floor. One attempt, and only one, had been made to remove it, and that attempt had made clear that it was either still attached to the immense geode wall meters below all the dirt, or else it was so large that it might as well be. So, when the farm had been laid out, it was lopped off level at the top, slightly too large for a stool and too small for a table, but still convenient to set tools and other things on as needed. A thermal blanket from pony ship insulation, hung over the entrance.
 * Tangled Hallway Best guess: About 20m deep.

A small chamber with crowded with lots of very large crystal shafts that stretch wall to wall, requiring carefull stepping around or climbing over. The huge crystals for the triple-sized batteries where cut here, opening up some space.   With large pockets which were once separate geodes containing various different colors of quartz
 * Third chamber: Lunch Buffet   about 13m average (irregular, widest point at least 21m) X 133m

Broken pillars line the walls Not a chamber, just a very narrow double-S-curved hallway studded with little crystals No pillars, almost spherical, full of large and mostly equal-sized quartz shading from clear through amethyst; its irregular bands of every color imaginable. ; at this point it has a few flaws in the perfection where Starlight has gone prospecting Another hallway full of small gems that makes an S-bend; fairly narrow. Constricted by some truly ancient collapse and reopening of the lava tube when it was still forming The innermost chamber, long and only a bit wider than Toothpaste Tube; the soil layer ends here, revealing bare crystal cave floor all the way back to a wall of crumbled granite (later sealed by Starlight).
 * The Crack Best guess: About 20m deep
 * The Orb 21m wide
 * Toothpaste Tube Best guess: About 20m deep
 * Bed of Nails Narrow, 110m deep (30m with soil, 80 without)

'' The following applies to the cave-system as a whole. To set the mood.''

''There were a couple of tight spots, but we were able to go back quite a long way- hundreds of meters, anyway. And let me tell you, it’s truly amazing. It looks like a gigantic geode that grew and absorbed smaller geodes. Most of the crystals are white, but there were a lot of yellow and red ones and even some purple. They come in all sizes, and I do mean all, from tiny enough to be set in a ring on up to shafts as big around as I am. ''

''We didn’t find any loose broken bits on the floor, but there’s a reason for that. The floor is hard-packed Mars soil, sloping down from the entrance. I have to bend a bit to get in without risking a scrape from the troll teeth, but ten paces in I can stand straight with no problems, and twenty paces in I can’t reach the ceiling. Apparently this cave or lava tube or geode or whatever it is opens to the surface periodically. Sand blows or falls into the hole until it fills, and then it hardens by compaction, leaving a brittle shell up top and opening a hole underneath. Eventually something happens- a meteor strike or a dust storm or something- the hole reopens, and the cycle repeats. And every time it does more sand slides further back into the cave, filling it in a bit at a time.''

''I don’t think the cave is really all one geode. There are places where the crystals change color slowly, like a Photoshop color gradient, but there are also tight points where the crystals are white on one side of the narrowing and orange or purple on the other side. And there are little pockets everywhere, sort of like cubbyholes, with other varieties of crystal. Best guess, a bunch of air pockets opened up in the molten rock, and what with one thing and another the bubbles grew together. Somewhere in the past the barriers between the geodes broke or something, and then mineral deposits sealed them together.

My light was good enough to see a gray wall barren of crystal at the far end, which I’m guessing is an old magma chamber.

Ignored Description:

And the water didn't swamp a huge area. The farm, remember, is TINY by Earth standards. They started with a heated area of ten meters wide by sixty meters long in an ovate chamber about twenty meters wide at its widest point and about one hundred forty meters long. A few little sinkholes of the kind described in the chapter, most of a night to fill them with water from broken pipes, and preexisting damage from the methane issue... well, it doesn't take much to wreak havoc with six hundred square meters of garden.