Mothermouth

Mothermouth
Mothermouth is a tunnel at HighStones that leads deep into the earth, inside it is decribed as "complete and utter blackness" and is very cold, cats must find their way through the tunnel by using their whiskers other than the sences they are more used to relying on, except if they are following their Clan Leader or Medicine Cat where they can choose to rely on their guides scent trail instead. The tunnel of Mothermouth ends at the cavern of the moonstone.

Spoiler Warning
Some cats fear being this far underground, such as Tigerclaw in Into The Wild when he enters Mothermouth with Bluestar and Firepaw. Firepaw can smell a subtle fear-scent coming from Tigerclaw and is confused by it. They have only just entered the cavern of the moonstone when Tigerclaw's fear-scent becomes overwhelming and he flees.