Once the scientists had their deep-mantle lava, they could study traces of undisturbed ancient water in basalt, lava's cooled form. The key to detecting where Earth's water came from lurks in its hydrogen atoms; specifically, in the ratio between two types of hydrogen atoms, called isotopes. Isotopes are identical, except for one key difference: the number of neutrons. One hydrogen isotope — deuterium — has one neutron, while another — hydrogen — has none.