Originally posted by ShaftAlmighty:

That circular stone staircase is nothing, just an aesthetic. As mentioned before, crypts are stone square structures the size of a small hut, lined with green torches. You need the swamp key so assume you killed the Elder already.

I strongly suggest mapping out the entirety of the swamp and mark on your map every crypt location first, then figure out a pattern of how you will mine them.

I think it may have been listed in a reddit thread, but it was a decent comment, so I'll add it.

Bring the swamp key with you on the scout run and unlock all the crypts in that swamp, then drop it at your base, so you've got one more open slot for hauling loot.