How have many limestone caves been formed?

Caves are formed by the dissolution of limestone. Rainwater picks up carbon dioxide from the air and as it percolates through the soil, which turns into a weak acid. This slowly dissolves out the limestone along the joints, bedding planes and fractures, some of which become enlarged enough to form caves.

Limestone caves:

A limestone cave can be defined as a natural cavity that is found under the earth's cavity.  It can extend to many kilometres in length and depth.

Limestone caverns are framed by the procedure of disintegration of limestone. Water alongside carbon dioxide air permeates its way through the dirt and changes into a frail corrosive. This feeble corrosive breaks down the limestone over some stretch of time and limestone caverns are shaped.

This gradually breaks up out the limestone along the joints, bedding planes and cracks, some of which become expanded enough to shape caverns.

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.