The most famous structures of La Palma are the "minas galerias" (water tunnels) which carry the water from sources in the mountains to cities, villages and farms (nowadays mainly banana plantations). La Palma receives almost all of its water supply due to the "mar de nubes" (sea of clouds), stratocumulus cloud at - altitude, carried on the prevailing wind which blows from the north-east trade winds. The water condenses on the long needles of the trees and other vegetation, it then either drips onto the ground or runs down the trunk etc., into the ground. Eventually it collects inside the rock-strata, and is then drained via the galerias into aqueducts and pipes for distribution. The galerias have been cut into the rocks over centuries. To visit the galerias a permit is required. It is possible to walk alongside many of the aqueducts, a popular activity for tourists (similar to the "levadas" of Madeira). The tour to the Marcos y Corderos waterfall and springs is also popular.