How are underwater caves formed?

Answer 1

underwater caves are formed when over a period of time, tides smashes the rocks and erodes it. gradually, it forms an underwater cave.

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It's true that sea-caves, as the answer states, are formed by erosion by wave-action, and they can hold deep water, but I suspect the questioner is asking about true underwater caves. If so:

Undersea cavessuch as the Blue Holes are normal terrestrial karst caves formed in low-lying limestone at a time of depressed sea-levels (the glaciations), then drowned as sea-level rose when the ice-cover retreated.

Water-filledcaves on land areagain simply normal limestone caves, but the passages have formed vertically-sinuous profiles that trap water in "sumps",rather like the U-bend under the kitchen sink.

Answer 2

If you mean those below the sea, such as the Blue Holes of the Bahamas, they formed in the normal ways above water when the sea-level was reduced by the Ice Age's glacial phases, then drowned as the sea rose in the subsequent warming.

However caves above the sea can develop water-filled passages called "sumps", such as most of the extent of those below the Nullarbor Plain in Australia.

The sumps in the remote parts of Wookey Hole (Somerset, SW England) dip well below sea-level even though the resurgence itself, the area you can visit as a tourist, is above present S.L. This is due to the cave's controlling local geology, in very steeply-dipping limestone, not sea-level changes.