That's because there is often water under the rock.
那是因为岩石下面经常有水。
2
Part of the limestone rock layer is permeated by water from below.
下面的水渗入了部分石灰岩岩层。
3
The water is usually naturally occurring groundwater that seeps down along fractures in the rock; less typically, the water is artificially introduced by being pumped down from the surface.