Nether underlava flush with the floor redstone activated BUD triple piston extender hidden entrance!!! Perfect for hidden bases, bunkers or hiding stuff! This is the most complex build I made by far.

I have uploaded a .nbt file. You can put this inside your world save, under /generated/minecraft/structures. Then using a structure block in game you can load the entrance. I water proofed the whole thing with tons of glass, I hope that helps. then just replace the top and the floor inside with your block of choice.

I realized that the previous build did not work in the nether due to the new fast lava flow in the nether, so this is an update.
Can't wait to play with wireless redstone with sculk blocks in 1.17!
Previous underwater/underlava hidden entrance video:
https://www.youtube.com/watch?v=RH2P3QNzeCQ&ab_channel=Reaven
https://www.planetminecraft.com/project/hide-your-secret-stash-underwater-lava-in-minecraft-1-16-3/

Intro song:
TOKYO MACHINE & Guy Arthur - GET UP [​NCS Release]
www.youtube.com/watch?v=HV7mLcsUp5U
Outro song:
Vacation - Markvard
www.youtube.com/watch?v=nxYoIurjLX8