Red volcanic crater, green lake, Danakil Depression, Ethiopia

A prodigious pit

Without these jade waters as a backdrop, you might think we're seeing the site of a meteorite strike on Mars. Nabiyotum Crater has no connection to outer space, though,as its concave cone is the remnant of a collapsed volcano. It's a remarkable adornment to the southern end of Kenya's 180-mile-long Lake Turkana—the world's largest alkaline lake. Set sail northward and you'll pass another volcanic crater, the still-active Central Island, before reaching Ethiopia near the lake's top tip. But look out! Turkana's plethora of wildlife includes a healthy faction of Nile crocodiles.