Requirements
Method
Enter the Thais Sewers near the depot. You will face Rats here. Navigate to the south-west end of the cave (here). Use a pick on the tile 2 sqm north of the dead end on the eastern side. A hole should open. Go down, kill the 4 Cave Rats and get your reward in the Pile of Bones (Quest).
Enter one of these Sewer Grates.
Follow the green line to the south-west of the sewer.
Use a Pick on the tile.
Use the Pile of Bones (Quest) to get the Battle Axe.