Isla de Ometepe is an island formed by two volcanoes rising from Lake Nicaragua.

Both volcanoes, Concepcion  and Maderas, are joined forming one island.
Ometepe  is 31 km long and 5 to 10 km wide.

We couldn't describe it better than Mark Twain:

"Two magnificent pyramids, clad in the softest and richest green, all flecked with shadow and sunshine, who summits pierce the billowy clouds. They look so isolated from the world and its turmoil, so tranquil, so dreamy, so steeped in slumber and eternal repose."