How do underground cause caves to form?

Answer 1

Do you mean "How do caves form"?

For a start the adjective is superfluous. Caves ARE underground by definition!

The vast majority of the world's caves are in limestone upland where the depth of the individual beds and the tension-joint structures within the rock, together with surface topography and drainage (of rain and snow-melt), combine to permit initial movement of water from sink to rising (spring) though the joints and bedding-planes, faults and other discontinuities.

Limestone is soluble in ground-water, thanks to slight acidification from rain absorbing atmospheric carbon-dioxide as it falls, and over time the initial conduits enlarge, start to capture each other and eventually create stream-courses - cave passages.

Othe rocks can hold caves:

Lava tubes under basalt lava flood develop when still-molten lava flows from under the cooled and solidified crust.

Sandstone in deserts can hold rock-shelters scoured out by wind-blown sand.

Sea-caves: simply erosion features in cliffs subject to wave action and sub-aeriel weathering.

Talus "caves": somewhat stretching the point, these are cavities between fallen boulders and their source mountain-side of sea-cliff.

Mass-movement caves (aka gulls / gull-caves, slip-rifts): fissures created behind valley walls and sea-cliffs by the outer "skin" of rock translated by mass-movement creep.