Isla de Ometepe , Nicaragua

Island Ometepe is a true wonder of the Nature formed by two volcanoes rising from Lake Nicaragua. Its name derives from the Nahuatl words ome (two) and tepetl (mountain)...

One island thus, but two volcanoes: Concepción, the highest one and Maderas, which we climbed to reach the muddy lake formed in its creater. The two volcanoes are joined by a low isthmus to form one island in the shape of an hourglass.

For more info: Ometepe on Wikipedia