I saw a dream that I was one of the 30 dwarves living in a fortress. We had built an amateurish wooden structure aboveground from scrap boards, but it didn't matter because the main fort was naturally underground. While excavating more livingspace to the east, we discovered an underground cavern with human-built structures. There was a modern day apartment building and a portion of a huge concrete dam buried by time.
From the house we discovered amazing artifacts like plastic canteens, disposable razors and household electronics which were perplexing enough even without power to use them. As for the dam, I guess someone found a lever to pull...
A small, wide hatch opened at the bottom of the dam, and I heard water gushing forth from it. It's a good thing I started running in time, because soon the entire side of the dam burst open, smashed by a huge wave of water. It hit the building, collapsing it and killing many dwarves hauling inside, but by the time the water reached the fort its pressure had lowered enough to merely flush me onto the central dining room floor.
About two thirds of the dwarves had died on that incident and some considered abandoning the fortress, but I convinced them that we could just expand to the west instead. Besides, I still considered this site too valuable to lose because of the human ruins we had found. When the life had returned to mostly normal, I occasionally snuck out to the now wrecked building to bring in whatever artifacts had survived its collapse. I also found wide, round tunnels with black and yellow paintwork snaking further into the ground, but there were mechanical-looking four-legged spiders in the darkness so I didn't dare to go there alone.