Cave
A tsunami is the same as a cave. Because, if a wave is big enough, it will curl over on itself, creating a sort of cave within the wave. That's why it's the same as a cave!
どう
ほら
None
Best way to remember the reading for this kanji is to think about how the shape of a cave is the shape of a doughnut (どう).
An update is needed before you can continue but you have more than one WaniKani tab open.
Finish up what ever you are doing on the other tab(s), then close them to continue.
The update will complete automatically once the other tab(s) are closed.