The Brine Rat Cavern, also known as Sven's cave, can only be accessed during and after Olaf's Quest. Players can enter the dungeon by digging south of the windswept tree, which is on a plateau east of Olaf Hradson and south east of the Rellekka Hunter area. Players must have a spade in inventory to dig into the dungeon, each time they want to enter it. When the hole is dug, the player falls through it and is stunned for a few seconds, but no damage is taken. The player cannot exit the cavern from this point.
The south-western part of the cavern contains the exit back to the surface world. To exit, the player must roll back the rock to expose a cave leading to the outside. After exiting, the rock rolls back to block the cave, so the cavern cannot be entered by this route.
The cavern actually consists of the main cavern, an outdoors area on the coast, a slippery walkway, another outdoors, coastal area, and a small shallow cavern south of the second coastal area. Neither of the coastal areas appear on the world map, so the entire complex is considered a single dungeon.
The Brine Rat Cavern is in the north-eastern region of the Fremennik Province. This is a remote area and takes some time to walk to. There are some faster means to get here:
- Fairy ring code DKS is very close to the cavern's entrance. Use of the fairy rings requires players to complete at least part of A Fairy Tale Part II quest.
- The Eagle transport system flies to the Rellekka Hunter area, which is north of the dungeon's entrance. Players must have completed the Eagles' Peak quest to use an eagle to fly here.
- The passage to Keldagrim is close to the cavern's entrance. Players must have at least started The Giant Dwarf quest to have access to Keldagrim. Keldagrim is important in that two transport systems go there:
- The mine carts run to Keldagrim from three locations. One is the tunnel under White Wolf Mountain; access to the tunnel requires completion of the Fishing Contest quest. The second location is in the Dwarven Mine under Ice Mountain. The third is a hatch in the north-west part of Grand Exchange. All carts are free of charge.
- A train system runs between Dorgesh-Kaan and Keldagrim. The train is free to use but requires the player to have completed Another Slice of H.A.M. quest.
- The slayer ring teleport to the Fremennik Slayer Dungeon is just south of the Brine Rat cavern.
- The use of an enchanted lyre will teleport the player outside the gates of Rellekka, just south of the cavern entrance.
The cavern is somewhat remote from a bank. The bank in Keldagrim is physically the closest. A player with access to the fairy rings will likely find the bank in Zanaris more convenient. Otherwise Etceteria, Neitiznot, Jatizso, and Seers are the closest banks.
A tour of the cavern
The cavern has the following features:
- 1. Entrance from the outside world. You cannot return to the surface here.
- 2. Exit to the outside world. There is a cave passage behind a rock here. You exit the cavern by rolling the rock out of the way and then using the passage.
- 3. Giant bats
- 4. Brine rats - These rats are not aggressive and have some good drops, including many death runes, noted raw sharks, noted raw lobsters, the rare Brine sabre, and other items.
- 5. Skeleton fremenniks - These are level 40. Skeletons here or in Area 7 can be killed to obtain a key to the gate.
- 6. Picture door. During Olaf's Quest, players must solve a picture puzzle to open this door. Afterwards, players can open the door using a right-click option.
- 7. First coastal area. This is outdoors but does not appear on the world map. Skeleton fremenniks, levels 50 and 60, are here, as is one Rock Crab. Skeletons here or in Area 5 can be killed to obtain a key to the gate. Players can collect rope and rotting barrels here. (A player can quickly collect a full inventory of rope, but the area's distance from a bank makes it inconvenient as a free rope source for most purposes.)
- 8. Slippery wooden walkway. This connects the two coastal areas. A good Agility level helps in crossing the walkway, as does having nothing held in your hands (no weapons, shields, etc., equipped). Possibly, walking (not running) one square at a time across the walkway may help. If you slip, you will fall into the sea and be washed up on shore in the outside world, near the exit point of the cavern. The walkway does not become safer after the quest.
- 9. Gate - When travelling west-to-east, this gate must be unlocked with a key dropped by a Skeleton fremennik you kill. Clicking on the gate calls a puzzle screen where you must match the shape of the key to correct lock. The key can have five different shapes. Unlike the picture door, even after the quest you must solve this puzzle each time you want to open the gate from the west. The key disappears when you unlock the gate. You do not need a key to open the gate when travelling in the other direction (east-to-west).
- 10. Second coastal area. This has level 60 Skeleton fremenniks and another rock crab. The wreck of Ulfric's ship is here, as is Ulfric's gravestone marking his grave. During Olaf's Quest, a chest is also here, and opening it triggers the climactic battle between you and Ulfric's skeleton. The back (south) of this coastal area turns into a small, shallow cavern.
- The cavern is extensively featured in Olaf's Quest.
- Ulfric, an undead Fremennik captain, who only appears in (and is fought during) Olaf's Quest.
- Skeleton fremennik (levels 40, 50, 60)
- Brine rat (level 70) - The cavern is named after these beasts, which drop the rare Brine sabre.
- Rock Crab (level 102)
- Giant bat (level 64)