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 or steel walls partially or completely under a mountain that contain at least one ancient cryptosleep casket and can spawn multiple or none of the following: 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. The only way to find out what is inside is to deconstruct a wall section or blast a hole in. When selecting a character to deconstruct, make sure to toggle its mode from Attack to Flee or it may engage hostiles instantly against your will.
Ancient Cryptosleep Caskets
Each casket contains either a human belonging to the spacer faction, a spacer and a megascarab, a group of megascarabs, or nothing. The humans can be neutral or hostile, and can also spawn wounded, downed or dead. All caskets open simultaneously when any one of them is manually opened or attacked. Once opened, those capable of walking may start to wander around, others will try to escape to the map edges or attack your colonists. Anything living that emerges will be afflicted with cryptosleep sickness. If captured, these people typically have a low recruitment difficulty.
These spacers can spawn with synthread and hyperweave clothing or may be naked. They can be equipped with a weapon, and sometimes come with plasteel, components, and gold which drop when they are downed, captured or stripped. Characters recruited will be added not last in the colonist bar as with common recruits but, after your starter colonists and before any other incorporated later on to the roster.
These caskets can be claimed and used later by your colonists. Deconstructing them yields small amounts of the rare resource uranium.