Washout

Washout opens a passage through stone — for a while, with no pickaxe and no trace, and underwater it leads you dry. It is Hollow on a lens: the blocks along the strike step away, and a passage as tall as a person opens in the stone — three, five or seven blocks deep. After twelve to twenty seconds it closes from the mouth inward: everything that was there comes back.

HollowFray

Water does not flow into a washout — it stands around the passage as walls, and so you can walk dry under water. The passage waits until you step out of it. Washout does not go through chests, doors, torches or anything unbreakable.

HollowTide