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Adding to the hydraulic power of the waves is the abrasive force of suspended sand and rock.
Most sea-cave walls are irregular and chunky, reflecting an erosional process where the rock is fractured piece by piece.
However, some caves have portions where the walls are rounded and smoothed, typically floored with cobbles, and result from the swirling motion of these cobbles in the surf zone.
True littoral caves should not be confused with inland caves that have been intersected and revealed when a sea cliff line is eroded back, or with dissolutional voids formed in the littoral zone on tropical islands. Carbonic and organic acids leached from the soil may assist in weakening rock within fissures.
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.
The nature of the zone of weakness itself is surely a factor, although difficult to quantify.
A more readily observed factor is the situation of the cave’s entrance relative to prevailing sea conditions.
The Blue Grotto of Capri, although smaller, is famous for the apparent luminescent quality of its water, imparted by light passing through underwater openings.
The Romans built a stairway in its rear and a now-collapsed tunnel to the surface.
Located on New Zealand's Otago coast on the South Island, Matainaka Cave has proven to be the world's most extensive at 1.5 km in length.