But Bridenstine doesn’t know that—not for sure. No one does.

Out in the cosmos, water is actually everywhere, usually in the form of ice. Its signature has been found beneath the surface of Mars, in the atmospheres of exoplanets, and inside dusty interstellar clouds. It is not strange to find water beyond Earth, though our planet, cloaked in oceans, certainly wears it best.

The moon had a dry reputation from the very beginning. According to the leading theory, it formed from the debris of a collision between Earth and a Mars-size object about 4.5 billion years ago. The impact was so fiery that scientists suspected that any water, whether it came from Earth or the mystery object, would have boiled away for good.

But in the late 1990s, an orbiting spacecraft flew over the poles and detected an abundance of hydrogen, which, when combined with oxygen, forms water. The hydrogen felt like a bread crumb beckoning scientists to follow. The moon’s axis has a very small tilt, which means sunlight never reaches some polar regions. The cold and dark conditions, some scientists predicted, could protect any water ice from the sun’s destructive glare.

Read: Why the far side of the moon matters so much

Scientists tested this theory, a decade later, with force. The Lunar Crater Observation and Sensing Satellite (LCROSS) arrived at the moon in 2009 with a rocket booster, now empty, that had helped launch it into space. Hurled down to the surface, the projectile exhumed those grains of pure water ice, hidden in darkness for perhaps billions of years, and lofted them into the light.

Other experiments around that time provided more evidence of lunar water. Scientists detected hints of water in Apollo samples of volcanic glass, a remnant of the moon’s fiery beginnings. One astronomer, poring over images of the south pole, noticed that some spots on the surface shared similarities with minerals that require water to form. Even Cassini, a mission bound for Saturn, caught something. Cassini had turned toward the moon—a nice dry target—to calibrate its instruments on its way out and picked up some contamination that later turned out to be a signal for water. It was becoming very difficult to ignore the new story of the moon.

Today, scientists believe the moon harbors water inside and out. There is likely an ancient reservoir deep in its interior, and when the moon was young and molten, water escaped through volcanoes, froze in the vacuum of space, and rained down on the surface in beads of glass, the kind that astronauts later collected. The glass is found beyond the moon’s polar regions, but the trapped water is difficult to extract, says Anthony Colaprete, a planetary scientist at NASA’s Ames Research Center who led the LCROSS mission. “The glass needs to be heated to very high temperatures to drive the water out,” Colaprete says.