As a volcanically island geological activity is not uncommon and should by that logic create some rather amazing sights.
Googling underwater cave you realize that these water ways might strech across the entire island and would allow the water tribe ( That can hazzardlesslys swim all across the island ) to gain uniuqe feature. and or Tactical advantage… Regons being outside their reach would be Ihu as the water would be Frozen, and Mangai that instead of somewhat harmless water holds lava… not a thing I remember any matoran casually taking a swim in.
The Deserts too have them they are just deeper. and probaly has started some issues with the Local mafia… but They are justicars… they should be able to handle a few mobsters.
Other tribes that can’t swim or even hold their breath will mostly be unable to use this.