Conditions on the surface of Mars are much closer to habitability than the surface of any other known planets or moons. Other planets such as Mercury has extreme hot and cold temperatures. Venus is very hot and all other planets and moons are very cold. There are some natural places on Earth, where humans have explored, that are similar to the conditions on Mars. The highest altitude reached by a balloon that was carrying humans, was 34,668 meters (113,740 feet), a record set in May 1961. The pressure at that altitude is about the same as on the surface of Mars. Extreme cold in the Arctic and Antarctic match all but the most extreme temperatures on Mars.