No. 96: Nov-Dec 1994
Our long-time impression has been that our moon is a cold body, solidified eons ago, when its primordial ration of heat radiated away. But the lunar satellite Clementine -- tracked with great precision by lasers on earth -- undulates suspiciously as it orbits the moon.
"The overall shape of the orbit traces the broad tidal bulges raised on the moon by Earth and the sun; the size and timing of the bulges depend on the moon's rigidity. The Clementine data show that somewhere, probably deep in its interior, the moon is not quite as rigid as solid rock would be. Most likely, part of the rock is still molten."
(Kerr, Richard A.; "Clementine Mines Its First Nuggets on the Moon," Science, 264:1666, 1994.)