A lot of different iron farms exist, but most of them are big ugly boxes in the sky. Now to use those same designs underground, you would need slots cut all the way to the sky, which would kind of defeat the purpose of putting them underground in the first place.

So I came up with this new iron golem farm design, which can be place both underwater and underground, with only 2 1x1 airshaft going to the surface, both of which are easily hidden. And still the iron farm is just as efficient as the ones built in the sky.

They are also much easier to build, because you don't have to go up precariously in the sky, nor have to find some way to transport villagers up there, and you don't need many materials because you are digging it out instead of building it up, so this is easy to build in survival before you build your base, allowing you to farm iron while building.

Showcase:

Nearly invisible underwater shaft using ice:

Village doors principles:

The actual Iron Farm tutorial:

Showcase of villager spawning center:
Coming soon