Letheia Facility: Secrets

You can find a few extra chests inside a hidden room in the Letheia Facility, but the most important secret is the Svetlana Twofeather room. There, you can find a unique augment: the Resistance Augment. You will need some kind of explosives to reach one of the switches required for Svetlana, and poison immunity is also recommended.

Hidden Room

Considering how poor the loot of this room is, the pressure plate to open it is hidden way to well. Before entering the second building, in the area with the poison vat, you can find the pressure plate on the ceiling above you. You will need to use a Spike Sphere tech to reach it, or some way to fly like a rocket spear.

Then head to the elevator to the right leading you to the two consoles. Exit on the first floor and you will find a conduit open above you in the room with the poison pool. Use a Spike Sphere tech to climb up and you will find a mere 2 Ancient Essence chests.

Svetlana's Room

To reach Svetlana, you will need to find and activate 4 Gem Switches hidden in the dungeon.

First Switch

Reaching the first Switch will require you to do a whole extra part of the first building of the Facility. Before exiting the building, use the elevator to get in the upper floors. Head to the left and you will reach a room with a lava pool and a few blocks making a bridge above it. Those blocks are not protected, so they can be mined out or destroyed by explosive weapons. Once you are in the middle or the room, 2 Omnicannons will spawn on the ceiling and 2 Letheia Corp Spider will spawn on the ground. The explosion on death of the Spiders will destroy the floor, so be careful to not fall into the lava if you don't have lava immunity. The room to the left works the same way: Letheia Corp Spiders and Twiguns will appear as you make your way through the room and the floor can be destroyed. The exit is on the top of the room and you don't have to kill everything to open it. Pretty much as soon as you jump, 2 Twiguns and 4 Spiders will attack you, and when you reach the upper part of the room, 4 more Twiguns and 4 Spiders will appear.

Once you reach the room above, 2 Spiders will attack you supported by a Heavy Drone. The room to the right contains a few extra chests guarded by 2 Omnicannons and 2 Guards will spawn on your way back. The path to your left lead you to an elevator and a room to the left. The elevator lead you to a small area with 2 Guards and some minor loot and will also make 2 additional Guards spawn at the bottom of the elevator. The important room is to the left of the elevator. Dispose of the 2 Guards inside the room. In the top right of the room, you can find a small pressure plate on the ceilling. Activate it and head back to the exit of the first building. You will be attacked by 2 more Guards after the lava pool rooms.

The small tower to the right of the first building is now open. Inside, you will find 3 Guards and the Gem Switch on the top floor.

Second Switch

The second Switch is located in the second building. When looking for the switches for the elevator, head to the room with the poison pool in the first floor. Jump inside the poison pool using the right most trapped door and use your Sphere to reach a small underwater room with the second Gem Switch.

Third Switch

The third Switch is located on the second floor when looking for the elevator's switches. After the room with the lava pool, climb down the stairs and you will reach a Red Door. Blast it open with explosives and you will find the third Gem Switch behind it.

Fourth Switch

The last Switch is located after the locked elevator. After unlocking the elevator and dealing with the room with the 2 Heavy Drones, head to the left and you will reach a room with 6 buttons on the wall. You will need to activate those buttons in the correct order to open the room with the Gem Switch. The order is 4-1-2-6-3-5 from left to right. This order can be found inside a room in the first floor when looking for the switches for the elevator. You can find a room with 6 wall lamps that light up in this order.

Reaching Svetlana

Once all 4 switches are activated, simply head to the last room of the dungeon. You will find the door to the right of the teleporter open and Svetlana wait you behind the door. You will find 3 chests in the room, the middle one containing the Resistance Augment, aswell as 4 Ancient Essence chests.