When the Apollo 11 astronauts took humanity’s first otherworldly steps into the Sea of Tranquility, they traversed oceans of dry, powderlike rock, not water. The moon’s interior was thought to be bone dry until 2007, when water molecules were first discovered in lunar rocks. Since then, additional studies have found evidence of H2O and its building blocks hydrogen and hydroxide in lunar meteorites and Apollo-era rock samples. Online this week in Nature Geoscience, a team of researchers examines these studies and reports that water molecule concentrations vary by a factor of up to 100 between rock types, suggesting that the moon’s innards contain wet and dry patches. The most waterlogged rocks, such as the glass beads pictured above, contain only as much water as the driest part of Earth’s mantle, the team found. The researchers suggest that the lunar water’s scarcity and uneven distribution arose during the moon’s formation from a hot mess of gas and molten rock 4.5 billion years ago. They believe further lunar rock studies could help scientists better understand how the moon formed and why it’s so arid compared with Earth.