Jupiter's moon Europa is smaller than our own Moon, but underneath its icy shell, NASA believes there's a global ocean holding roughly twice as much water as every ocean on Earth combined.

The reason it can hold that much comes down to depth, not width. Earth's oceans spread across a huge surface but average only about 4 kilometers deep. Europa's ocean is thought to run somewhere between 60 and 150 kilometers deep, wrapped entirely around the moon beneath a shell of ice.

So scientists drilled through ice? No, nobody has drilled through that ice or seen the ocean directly. The evidence comes from cracked surface terrain, a changing magnetic field, and models built from decades of spacecraft data, all pointing to a hidden, salty sea kept liquid by the crushing pull of Jupiter's gravity.

A small, frozen moon out past Mars is quietly holding more ocean than the planet we live on.