Working out how to investigate the Tadpole Pens in Subnautica 2 is the game's first proper puzzle, as you venture into the area's Hot Caves to recover the door code that'll let you inside. NOA doesn't seem to send you to the Tadpole Pens until you've found the other black boxes, at least that's how it was for me.
This is a significantly longer trip than finding any of the previous black boxes, so stock up on the usual water, food, batteries, health kits, and maybe bring a distraction flare just in case. All that said, here's how to reach the Tadpole Pens, as well as the Zip black box location so you can open the door inside.
