Is there really water ice on the Moon, and how did probes find it?

Yes, there is. Probes have found evidence of water on the Moon, especially water ice in some permanently shadowed craters, and in 2018 water ice was confirmed in multiple locations. But it is not what the word ice might suggest. The ice is not in sheets on the surface or just under it. There may be small chunks, under about 10 centimeters, mixed into the lunar soil, and some water is chemically bonded with minerals.

That is a surprise, because after the Apollo samples the Moon was believed to be generally anhydrous. Any water vapor on the surface would be broken apart by sunlight, with the hydrogen and oxygen lost to space. The loophole is the shadows. Near the poles, some crater floors are in permanent shadow, and the poles are thought to act as cold traps where vaporized water collects, because they never reach high temperatures.

Proving it took decades of tries. Researchers at Caltech first suggested ice in polar craters in 1961. A 1994 radar test by the Clementine probe was inconclusive. In 1998 Lunar Prospector found extra hydrogen at both poles. In July 1999 NASA deliberately crashed that probe into a south polar crater, hoping to liberate water, and telescopes saw no water signature.

The turn came in 2009. In September, NASA announced that its M3 instrument aboard Chandrayaan-1 had found hydrogen, as hydroxyl bound to the soil, over large areas of the surface. In October, a rocket stage and then the LCROSS spacecraft hit Cabeus crater. A later definitive analysis found the water concentration in the plume to be 5.6 ± 2.9 percent by mass. Why does it matter? Large quantities of lunar water would make living there more cost-effective, since carrying water from Earth would be prohibitively expensive.