Ancient shrine
Having a colonist walk by the walls of the structure will trigger an Ancient danger event prompting:
"As (Colonist Name) draws near the ancient wall a sense of foreboding overcomes him. (S)He isn't sure why, but (s)he feels this dusty structure may contain great dangers"
Ancient structures are large rooms built of stone walls that contain at least one ancient cryptosleep caskets and can spawn multiple or none: artifacts (psychic animal pulsers or psychic insanity lances), bug hives, luciferium or mechanoids. Due to being fully enclosed, one cannot see within the room, and hovering over the grayed out area as seen in the picture will only say 'Undiscovered' in the bottom left corner.
Ancient Cryptosleep Caskets
Each casket contains a human belonging to the spacer faction. The human can be neutral or hostile, and can also spawn downed or dead. All pods open simultaneously when ordered to. Once opened, some will start to wander around, others escape to the map edges or converge on your colonists.
Pod people wear synthread and hyperweave clothing. They can be equipped with a weapon, and sometimes come with plasteel, components, and gold, which drop when they are downed.