The island's name, Marchena Island, comes from the Spanish monk, Frey Antonio de Marchena. It has an area of 130 km² and a maximum altitude of 343 meters. The island is not set up for visitors, although the surrounding water is used by aquatic divers on organised tours.
People generally see the island is they sail around the northern part of Isabela on the way to Tower Island, its nearest neighbor about 45 miles due west.
Like many of the Galapagos volcanos, Marchena has a caldera. Marchena's caldera is roughly elliptical and measures 7 km by 6 km, within the range of caldera sizes of the large western volcanoes. Marchena's caldera is unusual, however, in that it has been almost completely filled with young lavas, some of which has spilled over and down the sides. The oldest lavas are 500,000 years old.
Another reason why Isla Marchena is so famous is that, although it is uninhabited, it was embroiled in the ‘Floreana Mystery’. Here the dead bodies of Rudolf Lorenz and the captain of the ship he was on, washed up mysteriously on the shore of the island and were mummified naturally as there were no natural predators found here.