Horizon Zero Dawn's Deep Secrets of the Earth is the next mission after To Curse the Darkness, which saw you take down your main obstacle, the Eclipse Network.
Now it's time for some exploration of an intriguingly named facility, the home of Project: Zero Dawn.
If you're after help for other parts of the game, consult our complete Horizon Zero Dawn walkthrough and guide.
How to reach the Zero Dawn facility
Soon after the start of this quest, you'll be faced with a no-turning-back moment. The entry to the Zero Dawn facility is a one-way trip, so be sure to stock up before going in, and prepare yourself not just with the usual equipment like traps and potions, but maybe even some tougher armour and weapons, too.
Go to Sunfall - Follow the waypoint as usual.
Go to the Citadel Courtyard - Once you're there, continue to follow waypoints.
Enter the Citadel - And again... watching the cutscene inside the Citadel
Climb down - It turns out there's a secret entrance into the important facility just down the edge of the citadel. It's a one-way trip once you're inside though, so make sure you prep first, before hopping down off the Citadel steps, across some rocky pillars, down a wall, and through the door there into the cave.
Explore the Project: Zero Dawn facility - Once inside, it's time to explore. Take a look around, but basically it's just following your waypoint once again until you reach a purple circle on the floor, which plays you a hologram.
Enter the Viewing Room - Once that's finished, enter the door behind the holorgam.
Kill the Shadow Carja - On the other side, some Shadow Carja have come down through their own entry points to investigate. Take them out however you like - they shouldn't be too troublesome.
Enter Viewing Room - With them down, follow the waypoint to the Viewing Room, but make note of any datapoints you find on the way.
Go to Processing - In the Viewing Room, a hologram of Elisabet Sobeck plays. Next up, Central Projects.
Go to Central Projects - Follow the waypoints again. Some heavy Carja enemies appear, but you can conveniently dip in and out of cover behind the doorway to pick them off with headshots before they mow you down.
Zero Dawn facility door puzzle
Restore Power to The Door - Door's locked, and requires you to solve those combination puzzles again. Follow the waypoint downstairs to a room with two sets of combinations. One goes: Left, Up, Left, Down, but the other is broken. Listen to a nearby datapoint for more info, which tells you to head up to a higher level to restore it. On the left of that room as you enter it, behind the wall with the solvable puzzle, is another door. Through there, crouch down and turn right, through a low vent, to another room that has the item you need to fix the lock. With that restored, the code, from left to right, is: Down, Left, Up, Right, Down. (The datapoint says it's backward, but that isn't awfully clear - it's double backwards, in both order and direction).
Enter Central Projects - Go back to the locked door and head through.
Kill the Eclipse - There are a few waves here, but they're still fairly easy to take out. Use fire arrows on the chaps with Blaze canisters on their backs if you need some extra help.
Find the Entrance to Sobeck's Office
Follow the waypoint through a series of rooms that house their own holographic datapoints, which you can opt to play for some extra story flavour if you so wish.
Eventually you'll end up on a platform above some more Carja, which you can follow round and down some steps to attack them from the side. Keep following your waypoint to an elevator shaft, which you can climb up via the yellow-gold ledges and ladders much the same as you've done in missions past.
Head round the side of Elisabet's office once you find it, watching the holograms of her inside, until you find the door in.
Examine Elisabet Sobeck's terminal - Activate the terminal inside her office for a cutscene, and a rude interruption.
How to beat the Behemoth
Lure the Behemoth into Ramming the Support Pillars - You'll wake up, without any equipment, in the arena of the Shadow Carja, with a big Behemoth staring you down.
Move quickly, and pause in front of the big pillars. As it charges, roll out of the way - a double roll is needed really - and it'll crash into the pillar. Do that for both of them and it'll take down the entire platform, granting you access to your gear in the process.
For more Horizon guides, explainers and advice, take a look at our main Horizon Zero Dawn guide and walkthrough hub, or our advice on finding Horizon Zero Dawn's override Cauldrons for controlling any machine, and unlocking some super-strong armour with our Horizon Zero Dawn Power Cell locations guide, too.
Kill the Behemoth - Now it's back to standard techniques for taking down the machine. The canisters on its back are relatively easy to pick off for decent damage, and respond well to frost-based attacks. Don't forget to use your rope- and tripcasters, too, to help yourself out.
After you've defeated the Behemoth, humiliated the Shadow Carja, and been rescued at last by an in-person Sylens, there's no time to lose in getting on to your next mission and rescuing your tribe, in The Heart of the Nora.