Marchena's 343m high shield volcano has been very active with an eruption in 1991, so about three quarters of the island are covered with black lava and there are several fumaroles (steam vents).

It is very dry and hot here. Except for diving it is seldom visited. You dive at Punta Espejo. Underwater you might see cow-nosed rays, turtles, schooling hammerheads and schools of blue-striped snappers, grunts, surgeonfishes, spotted moray eels and scorpion fishes.

Here are fields of the endemic Galapagos garden eels. They are dark brown with a row of white spots on the sides of the body and some irregular patches. These eels inhabit sandy areas where there is some current, so they can feed on zooplankton, that drifts past their burrows. Like all garden eels they tend to be shy and retreat into their holes when approached. About 44cm. Also found in Mosquera, Plaza islands and Devil's Crown.