The Galápagos Islands first appeared on maps drawn by Flemish cartographers Abraham Ortelius and Gerardus Mercator in 1570. The Galápagos take their name from the old Spanish word for tortoise, as early visitors found giant tortoises roaming the islands. There are many other species that have made unique adaptations to the local environment, including the marine iguana. It is the only iguana to feed in the sea, diving up to 50 ft (15 m) to forage for marine algae. The cold waters of the Humboldt Current allow Galápagos penguins to survive at the equator. The islands are the result of volcanic eruptions above a mantle hotspot. The same hotspot is responsible for driving the Cocos and Nazca plates apart at the Colon Ridge. The Nazca Plate on which the islands sit is moving eastward, so the oldest of the islands are found in the east. They have been volcanically extinct for several million years, but some of the younger islands are still active volcanoes. Farther east, the submarine Carnegie Ridge is also built from Galápagos Hotspot material.