Originally posted by warrenchmobile:

That is definitely not a Sunken Crypt. The structure pictured is mostly found in the Mountain biome. It has a stone staircase winding down to the bottom where there is a paved floor and possibly a chest and possibly a vegvisir. Vegvisirs are carved stones revealing a boss location.

It may also be found in the Swamp but fully or partially filled with water. One cannot access the flooded portion below one's chest because one cannot swim underwater. The Swamp Key is useless here since there is no gate to unlock.

I have never encountered one of these structures in a Meadows, Black Forest, or Plains but I haven't visited every location on the map. The original purpose of these structures remains a mystery to me.