We have known from past data and studies there is liquid magma underneath the Moon's surface, but were unsure why none have risen to the surface.
Now, due to recent findings, it is believed the reason for this is that the molten rock is extremely dense, thus preventing it to rise to the top.
X-rays illuminate the interior of the Moon
ScienceDaily (Feb. 19, 2012) — Unlike Earth, our Moon has no active volcanoes, and the traces of its past volcanic activity date from billions of years ago. This is surprising because recent Moonquake data suggest that there is plenty of liquid magma deep within the Moon and part of the rocks residing there are thought to be molten. Scientists have now identified a likely reason for this peaceful surface life: the hot, molten rock in the Moon's deep interior could be so dense that it is simply too heavy to rise to the surface like a bubble in water. For their experiments, the scientists produced microscopic copies of moon rock collected by the Apollo missions and melted them at the extremely high pressures and temperatures found inside the Moon. They then measured their densities with powerful X-ray beams.