Subnautica 2 has one major locked door puzzle in Early Access: the Tadpole Pens. Unlike the original game's fixed keycodes, this requires a multi-step quest chain.
1. "The Tadpole Pens door won't open"
The fix: There's no fixed code. Press the red button inside to trigger NoA. Then find Zip's Blackbox at 310m in the Geothermal Cave (southwest of the Pens). That grants the kuraltai access key.
2. "I can't breathe at 310m"
Build the Tadpole vehicle first (max depth 250m). Bring an Air Bladder and Standard Air Tank. Craft a Rebreather if possible. Don't explore — go straight down, grab the blackbox, ascend.
3. "NoA won't send the waypoint"
Exit and re-enter the area. Save and reload. Explore further away — NoA's dialogue sometimes queues behind other voice lines. Check your Signals tab.
4. "Found the blackbox but quest didn't update"
Make sure you actually interacted with it (not just swam near it). Check inventory for the kuraltai access key. If missing, reload and try again.
5. "Tadpole can't go deep enough"
Park it at 240-250m. Mark it with a Beacon. Swim the final 60m with Air Bladder + Air Tank. It's a small cave — you won't need to explore far.
6. "Can't find Tadpole fragments"
Look in and around the Geothermal Cave entrance area (150-200m). Also check near the Welcome Center (85m SE of Lifepod).