https://steamcommunity.com/sharedfiles/filedetails/?id=2811213512

I am not sure of their purpose but I have encountered what I call missile silos in the Mountain biomes and sometimes in swamp biomes. They are circular stone shafts with a stone staircase spiraling down the inner wall to a stone floor some 20 meters below ground level. Silos in the mountains may have a vegvesir at the bottom, or a chest, or nothing at all. Silos in the Swamp are water-filled.

I found one about 50 meters from a Moder summoning altar in the mountains. I guessed it might provide a safe place to rest while fighting Moder so I built a sheltered campfire at the bottom. What could go wrong?

Early on the following game day, I geared up with my draugr fang bow, obsidian arrows, wolf armor, banded shield, and silver sword. Grabbed a few health and tasty meads.

Summoned Moder and had her down to about 50% health when I decided to rest in the silo. Moder landed at the edge of the silo and attacked with her breath.

I was able to block most of the damage to my viking with the shield. The campfire was snuffed out and it's covering roof section destroyed but, worse yet, the section of stone floor on which I was standing collapsed. Silos have a sub-basement, at least this one did. I was left on snow-covered ground perhaps four meters below a largely intact stone floor. I could only see out if I looked up through the two-meter square section of the stone floor that had collapsed under me.

After a while Moder made her way to the bottom of the silo. I spent a couple of minutes shooting her from underneath the floor. She had only a quarter of her original health when she finally decided where I was and turned out the lights. I was dead in a heartbeat.

Round Two with troll armor, huntsman bow and poison arrows, bronze buckler and iron mace, and lots of health, tasty, and frost resistance meads. Killed a weakened Moder outside of the silo. Had to build a scaffold to descend to the bottom of the silo and retrieve my corpse. In the screenshot, one can see the outline of the original stone floor curving from the bottom right corner to the middle of the left side.

I found nothing at the bottom aside from my corpse and the stones left from the collapse of the floor. Now I want to find another silo and hack through its floor to see what may be underneath.