Geothermal Valley - Baths of Kitezh Challenge Tomb (Part 1)

The entrance to this tomb is through a cave at the north end of the valley floor. It is close to the waterfall and the Astronomer's Crypt, and it's marked with a BEAR icon on the map. As you'd expect, this clearing has the distinctive, barren look of a place that's been ravaged by a bear. And naturally, you'll need to defeat one in order to enter the tomb. She's big and tough, but she can be killed using the same tactics I've recommended before.

Prep as many poison and/or grenade arrows as you can carry, equip your bow, and enter the den quietly.

Peer around the corner to target the bear. (If you move out too far, she'll notice you.) Fire one or two poison arrows.

...

Then switch to grenade arrows for a quick takedown, or alternate one poison arrow and several regular arrows, or poison arrows and bursts of gunfire (as shown above. If the bear manages to shrug off the poison and charge at you, the bow becomes unwieldy, so switch to the shotgun and blast away.

NOTE: Even if you killed this bear earlier, take care when entering the cave. The bear does respawn, but I'm not sure whether the interval is scripted or random.

When the bear is dead, harvest the bear skin.

Gather some mushrooms and magnesite, as well as deer hide from the bear's kill.

Then use your axe to break down the crumbling wall.

Follow the passageway to a small pool. With the aid of the REBREATHER, which you obtained in the FLOODED ARCHIVES, you can swim through the tunnel...

...into a marshy area with crumbling walls and hanging branches. As you approach the man-made archway on the far side of the pool, look to the right to spot a salvage crate sitting on a ledge. Get the goodies and then go through the archway.