Olympus Mons is one of a dozen large volcanoes, many of which are ten to a hundred times taller than their Earthly counterparts. If Martian space tourism takes off in the coming decades, Olympus Mons could become a prime destination for adventure enthusiasts. Olympus may once have been a volcanic island surrounded by an ocean nearly 4 miles (6 km) deep, according to geological evidence found in the high cliffs. Another new study found that the rover discovered polygonal wedges 35 kilometers underground – the first time they have been discovered beneath the planet’s surface. Evidence suggests that the wedges formed between 3.7 and 2.9 billion years ago due to major climate changes in Mars’ past. The Mars Express orbiter has discovered enough water ice buried beneath the Red Planet’s equator to cover the entire planet in a shallow ocean if it melted.