Sea cave

Some of the largest wave-cut caves in the world are found on the coast of Norway, but are now 100 feet or more above present sea level.

By contrast, in places like Thailand's Phang Nga Bay, solutionally formed caves in limestone have been flooded by the rising sea and are now subject to littoral erosion, representing a new phase of their enlargement.

The Blue Grotto of Capri, although smaller, is famous for the apparent luminescent quality of its water, imparted by light passing through underwater openings.

[2] Matainaka Cave – located on the Otago coast of New Zealand's South Island – has proven to be the world's most extensive at 1.5 km in length.

The latter may also occur in igneous rocks, such as in the caves on Santa Cruz Island, California, where waves have attacked the contact between the andesitic basalt and the agglomerate.

As the sea reaches into the fissures thus formed, they begin to widen and deepen due to the tremendous force exerted within a confined space, not only by direct action of the surf and any rock particles that it bears, but also by compression of air within.

Blowholes (partially submerged caves that eject large sprays of sea water as waves retreat and allow rapid re-expansion of air compressed within) attest to this process.

For example, sea urchins drill their way into the rock, and over successive generations may remove considerable bedrock from the floors and lower walls.

A more readily observed factor is the situation of the cave's entrance relative to prevailing sea conditions.

At Santa Cruz Island, the largest caves face into the prevailing northwest swell conditions—a factor which also makes them more difficult to survey.

Sediment dating in the largest of these (Halvikshulen in Osen, 340 m long) shows that it was formed over a period of at least a million years.

By contrast, caves formed along horizontal bedding planes tend to be wider with lower ceiling heights.

In some areas, sea caves may have dry upper levels, lifted above the active littoral zone by regional uplift.

There is an exceptionally large cave underlying the Fogla Skerry, an islet off the coast of Papa Stour, in the Shetland Islands.

Matainaka Cave in New Zealand has 12 separate entrances into which waves can penetrate and numerous joints along which intersecting passages have developed.