El Yunque is the only tropical rainforest in the forest system of the United States, in the mountains of eastern Puerto Rico, and up to six metres of rain fall on it in a year. That rain sorts the whole mountain. Tall trees and giant ferns fill the valleys, and the higher you go the smaller everything gets. On the ridgetops it ends in a dwarf forest of gnarled, moss-wrapped trees standing in drifting cloud, where the air turns cool and wet and the trail goes quiet. Route 191 climbs into all this between walls of bamboo and dripping green to the stone Yokahú tower, whose spiral stair puts you above the treetops with the Atlantic beyond. The paved La Mina trail drops along the river to La Mina Falls, where cold water pours over dark rock into a broad pool ringed with ferns and you can wade in under the spray. Away from the falls the same streams gather in shaded rock pools, tree-cooled and clear.
After dark the forest turns into one sound. The coquí is a frog the length of a fingernail that calls its own name in two notes, ko-kee. Thousands of them call at once from every leaf, so the chorus reaches you from every direction and has no centre. It never passes through a tadpole stage: the eggs are laid on land and each one hatches into a finished frog. Thirteen of Puerto Rico's sixteen coquí species live in this forest, and the island treats the sound as its own. Overhead is a bird nobody hears so easily. The iguaca, the Puerto Rican parrot, fell to thirteen wild birds, all of them here, and aviaries built inside the forest have been breeding and releasing them ever since. The mountain is named for its cloud twice over. The Taíno called the high peak Yuke, thought to mean the white lands, for the cap that almost never lifts off it; the Spanish heard yunque, their own word for an anvil, which is what that flat top looks like from the coast.