TL;DR

The Tadpole Pens are located roughly 675 meters east of the Lifepod at around 150 meters depth.
The locked door requires a keycode found in Zip’s Black Box — not inside the Tadpole Pens itself.
Zip’s Black Box is about 200-300 meters southwest of the Tadpole Pens, inside a Hot Cave at 310 meters depth.
You need the Tadpole submarine, the Rebreather, and the Heat Tolerance adaptation before making this trip.
The keycode saves automatically to your PDA. Just press the button at the locked door and it opens.
Inside you’ll find the Scout Ray Chassis blueprint, Moonpool Recipe Data Card, Repair Tool scan, and a NOA terminal that unlocks the next major story objective.

The locked door in the Tadpole Pens is one of the first real walls you’ll hit in Subnautica 2. You can see it. You know something important is behind it. But pressing the button just scans you and denies access, and the game doesn’t clearly tell you where to go next.

The answer is not inside the Tadpole Pens at all. The keycode is hidden in a deep volcanic cave system roughly 200 to 300 meters away. Getting to it takes real preparation. This guide walks through every step from finding the Tadpole Pens to opening the door and claiming everything inside.

Table of Contents

What Is the Tadpole Pens and Why Does It Matter?

The Tadpole Pens is a mid-game story location sitting roughly 675 meters east of the Lifepod at a depth of around 150 meters. It’s an industrial facility embedded in the ocean floor near heated cavern systems and volcanic vents. You’ll receive the “Investigate Tadpole Pens” objective from NOA after completing several earlier black box investigations.

This mission is a turning point in the game. Completing it unlocks major Tadpole vehicle progression, key blueprint scans, and the waypoint to the next major story area — the Alien Ruins. Many later objectives assume you’ve already been here. It’s not optional content. It’s a gateway.

The Collector Leviathan also roams this area, so approach with caution and don’t linger in open water longer than necessary.

What You Need Before Going to the Tadpole Pens

Do not attempt this mission underprepared. The keycode retrieval involves a 310-meter deep cave with heat damage. The following gear is required or strongly recommended:

Tadpole submarine — The Pens are far from the Lifepod and the keycode cave is beyond safe swimming depth. The Tadpole makes the journey manageable and provides unlimited oxygen while piloting. See our full guide on how to build a Tadpole in Subnautica 2.
Rebreather — Oxygen drains drastically faster below 100 meters without it. Below 200 meters, you can lose a full tank in seconds. The Rebreather removes this depth penalty entirely. You’ll need to exit the Tadpole to reach the black box, and the Rebreather is the only thing keeping your air manageable at 310 meters depth.
Heat Tolerance adaptation — The Hot Caves around the Tadpole Pens deal continuous heat damage without it. You cannot safely operate in this zone without it. See our guide on how to get the Heat Tolerance trait before attempting this mission.
High Capacity Air Tank — Maximizes your oxygen for the moments you’re swimming outside the Tadpole. Read our guide on how to increase oxygen in Subnautica 2 for the full upgrade path.
Air Bladders — Carry at least two or three. They give you emergency oxygen and a fast burst toward the surface when things go wrong. Cheap to craft and essential for deep cave dives.
Food and water — This is a long trip. Stock up before heading out. Check our guide on the best early food sources and how to make water so you’re not caught short mid-mission.
Repair Tool — The cave route is tight and your Tadpole will take bumps. Keep it ready. See how to get the Repair Tool if you haven’t already.
Distraction Flares — Optional but useful if you encounter the Collector Leviathan near the Tadpole Pens. Throw one in the opposite direction to buy yourself time to escape.

Step 1 — Reach the Tadpole Pens

From the Lifepod, head east for roughly 675 meters. The Tadpole Pens sits in the lava vents area at around 150 meters depth. If you have the “Investigate Tadpole Pens” signal active, follow the waypoint. If not, head east until you spot the large industrial facility structure embedded into the rock formations near thermal vents.

Once you arrive, swim inside through the open hatch. Before worrying about the locked door, explore the facility entrance thoroughly. There are scannable objects, data entries, and lore terminals worth interacting with. One terminal inside the right room will confirm that a colonist named Zip took the door code down into the Hot Caves. This is how you learn where to go next.

Also scan any Tadpole Fragments you find here — the Pens contains multiple fragments and several useful blueprints in the open area, even before you open the locked door. Bring your Scanner and don’t skip anything.

Step 2 — Get the Zip Black Box Signal from NOA

After exploring the Tadpole Pens entrance, NOA will eventually send you a waypoint to Blackbox — Zip. This signal marks the exact location of the black box that holds the door code.

If you want to wait for the signal before heading out, return to your Lifepod or any base with a NOA Terminal and give it a few minutes. Once the signal arrives, it marks the Hot Cave entrance on your HUD and makes navigation much easier.

If you don’t want to wait, you can head directly to the location from the Tadpole Pens without the signal. The route is described in Step 3 below. Either approach works, but having the NOA waypoint active makes the trip significantly less disorienting in the cave tunnels.

Step 3 — Find Zip’s Black Box in the Hot Caves

This is the hardest part of the mission. The black box sits at 310 meters depth inside a volcanic cave system roughly 200 to 300 meters southwest of the Tadpole Pens entrance. You cannot take the Tadpole all the way to the bottom — the submarine’s default crush depth is 250 meters. You’ll need to park it above that threshold and swim the final section on foot.

How to Navigate to the Black Box

Start at the Tadpole Pens entrance and head south-southwest, keeping close to the ocean floor. Stay on a bearing of roughly 195 degrees. You’ll move through a ravine with lava pipes above you. Keep going until you spot a cave entrance in the ground.

Enter the cave and look for a glowing blue tunnel entrance to the southwest. This tunnel takes you deeper. Follow it down until you reach a large underground chamber with a big lava vent at its center.

From the center of that chamber, go down and to the right. You’ll see a smaller cave opening on the right side of the chamber wall. This is where you need to exit the Tadpole. Park your submarine here — it will be crushed if you take it any deeper.

On foot with your Rebreather equipped, swim down through the cave ahead. At the bottom, look for a supply crate. Zip’s Black Box is sitting on top of it. Interact with it and the keycode saves automatically to your PDA. There is no code to write down or memorize — it stores itself.

While you’re here, you may also find Gram’s Black Box nearby. Don’t get distracted by it — focus on Zip’s first since that’s the one you need for the door.

Important Notes for This Dive

Exit the Tadpole at or above 250 meters depth. Going deeper in the vehicle destroys it.
The Rebreather is non-negotiable here. Without it, your oxygen depletes in seconds at 310 meters.
The cave is hot. Without the Heat Tolerance adaptation, you’ll take continuous damage and likely die before reaching the black box.
Have Air Bladders ready. If your oxygen gets critical, activate one immediately and begin ascending. Don’t wait until you’re at zero. See our guide on how to increase oxygen if you need more breathing room before attempting this section.
The cave entrance is marked roughly by the Zip Black Box marker on your HUD, but the black box itself won’t be visible until you’re inside. The marker is pointing at the cave, not the object. Go in.

Step 4 — Return to the Tadpole Pens and Open the Locked Door

With Zip’s Black Box data stored in your PDA, pilot the Tadpole back to the Tadpole Pens. Head to the locked door in the left corridor and press the red button on the panel in front of it.

Because the keycode is already saved to your PDA, the door opens automatically. No manual code entry required. The heavy door slides open and you’re in.

The room inside is breathable — you don’t need to worry about oxygen once you’re through the door. Take a moment to catch your breath and explore without the clock ticking.

What’s Inside the Locked Room?

The locked room is one of the most reward-dense locations in the early-to-mid game. Here’s everything you’ll find:

Scout Ray Chassis Blueprint

This is the biggest reward. The Scout Ray Chassis maximizes your Tadpole’s acceleration, top speed, and turning rate, making it far better for solo exploration and scouting. It attaches directly to the Tadpole via the Vehicle Fabricator. To craft it you’ll need 2x Plasteel Ingot, 1x Advanced Wiring Kit, 1x Dedicated Core, and 1x Strong Acid.

Strong Acid is processed from Necrolei Cysts — our guide on how to get Necrolei Cysts and Strong Acid will point you to the right location. For ingots, see our guide on how to build a Processor and craft ingots.

Moonpool Recipe Data Card

If you haven’t built your Moonpool yet, this card unlocks the blueprint. The Moonpool is the base structure your Tadpole docks with and is required to craft and store the vehicle. See our guide on how to build a Tadpole for the full setup walkthrough.

Repair Tool Fragment

A scannable broken Repair Tool sits inside the room. If you haven’t unlocked this blueprint yet, scan it now. The Repair Tool is essential for maintaining your Tadpole’s durability during dives. Read our guide on how to get the Repair Tool for more.

Additional Scannable Objects and Lore

The locked room contains multiple other scannable items tied to the Tadpole vehicle system and base-building progression. Scan everything before leaving. Missing a scan here can force you to return later.

NOA Terminal and Sophie Boucher

Climb the ladder at the back of the room to reach a second floor with a NOA Terminal. Power it on and exhaust all the dialogue options. This triggers a conversation with Sophie Boucher and completes the “Investigate Tadpole Pens” quest. It also marks the Alien Ruins on your map — your next major story destination. Everything after this point opens up from here.

Common Problems and How to Fix Them

The Button at the Door Won’t Work

You haven’t downloaded the keycode yet. The door button only responds after Zip’s Black Box data is saved to your PDA. Go back and interact with the black box in the Hot Cave first.

Can’t Find the Cave Entrance

If you don’t have the NOA waypoint active, the cave is easy to miss. Go back to your Lifepod or a base NOA Terminal and wait a few minutes for the Blackbox — Zip signal to transmit. Once it appears on your HUD, it marks the cave entrance. Don’t try to navigate the lava caves without that marker on a first attempt.

Dying From Heat Before Reaching the Black Box

You don’t have the Heat Tolerance adaptation active, or it isn’t unlocked. Do not enter the Hot Caves without it. Get the trait first. Our guide on how to get the Heat Tolerance trait covers exactly what you need.

Running Out of Oxygen at 310 Meters

You’re either missing the Rebreather or haven’t upgraded your Air Tank. At that depth without a Rebreather, a full High Capacity Air Tank can drain in under 15 seconds. Equip the Rebreather before leaving the Tadpole. Also carry multiple Air Bladders for emergencies.

Tadpole Gets Crushed

You went too deep in the Tadpole. Park it above 250 meters and swim the final section on foot. The game will warn you when you’re approaching crush depth — stop and exit the vehicle the moment you hear it.

Can’t Find the Tadpole Pens

If you haven’t received the “Investigate Tadpole Pens” signal yet, you may need to scan more objects in the world. NOA sends the signal after you’ve progressed through enough earlier black box missions. Keep scanning everything you find and NOA will transmit the waypoint when you’ve met the threshold. Use the Scanner Station to help track down items and progress more efficiently.

What Comes After the Tadpole Pens?

Completing the Tadpole Pens and talking to Sophie Boucher marks the Alien Ruins on your map. This is your next major story location and opens up the mid-to-late game progression path. Before heading there, make sure you’ve done the following:

Craft the Scout Ray Chassis and attach it to your Tadpole for faster travel.
Get the Depth Module Mk. I to push your Tadpole’s crush depth from 250 to 450 meters.
Stock up on inventory space — the Alien Ruins will have a lot to scan. Check our full guide on how to increase inventory space in Subnautica 2.
Top up food, water, batteries, and health kits. The Alien Ruins area is deeper and more hazardous than the Tadpole Pens.

For a broader view of where you should be in your progression at this stage, see our guide on the best early upgrades and progression path.

More Subnautica 2 Guides

Subnautica 2 is available now in Early Access on Steam and Xbox, including Xbox Game Pass.

Subnautica 2: How to Get the Keycode and Open the Locked Door in the Tadpole Pens

What Is the Tadpole Pens and Why Does It Matter?

What You Need Before Going to the Tadpole Pens

Step 1 — Reach the Tadpole Pens

Step 2 — Get the Zip Black Box Signal from NOA