Point of order, you 'need' the key to get in. There are several exploitative ways around that.

As others have said, the crypts are all there when the map is generated. You'll need the key (which you get from killing The Elder) to unlock the doors, and you only need to do it once on each crypt. Also, very rarely you will need it inside the crypt itself.

Originally posted by magilla666:

Also, very rarely you will need it inside the crypt itself.

That's so rare I'd forgotten it existed. Re: my other comment above they should probably add a lot more of that. Actually if the locked door was always inside that could close all the loopholes.

I always found enough in the boss’s swamp.

Just need the key to unlock them, everything, and I mean everything is rng for your seed. However, there are some generalities like large swamps have a larger chance of crypts to unlock and you only need to unlock it once. Smaller swamps might not have any crypts.

Just need the key to unlock them, the crypt locations are determined when the world is created. Not every swamp has crypts, some swamps have tons of them, just gotta keep looking.

bigger swamp areas have more crypts. if you have a swamp close to your spawn it might not be that big. a good way to find crypts is to sail around and you can see the crypts from the shore (green lights). many swamps border ocean water.

Originally posted by bacon:

bigger swamp areas have more crypts. if you have a swamp close to your spawn it might not be that big. a good way to find crypts is to sail around and you can see the crypts from the shore (green lights). many swamps border ocean water.

Bigger swamp area's can easily have one crypt and a smaller one 3/4. Its all very random.

Also the swamp is indeed often found bordering ocean but crypts may appear anywhere in the biome.