Originally posted by neuroperson:

Upgrade your pickaxe, and ideally bring 2 so you can get all the iron in a sunken crypt in one go. If I remember you should get 30 -60 iron per crypt.

One nice thing about the crypts is that a basic antler pickaxe works as well as any other on the mud piles, so you can build a workbench right by the door for quick and easy repairs as needed.