What is the importance of the Thunder Caves in the novel, Eaters of the Dead: The Manuscript of Ibn Fadlan?

The Thunder Caves are caves in the cliffs that can be reached either by land or by water and are the home of the wendol's mother. The land entrance is guarded by the wendols and the sea entrance is very risky. Buliwyf and his men enter via the sea entrance. They must swim against the sucking tide beneath the cliff in order to reach the caves. Once inside, the waves crashing outside sound like thunder and this is how the caves were named.