NASA is preparing to send two satellites to the Moon to take extremely accurate measures of the Moon's gravity:
What I cannot help but notice is that decades after we went to the Moon with Apollo, the mysteries of Lunar gravity are still being contemplated by NASA with its suggestively named "Grail" mission. The name "Grail" suggests the "holy grail" of physics, which was to find and plumb the nature of gravity and eventually to be able to control it. And by naming the mission "Grail" NASA seems to be suggesting that the Moon holds some sort of key to that mystery.
Indeed it does, and a brief review of the history of Lunar Gravity anomalies might be in order to explain why. In the heady days of early lunar exploration ending with the Apollo missions, scientists noticed first of all that there were areas on the Moon where mass seemed to be concentrated beneath the surface, causing noticeable local variations of gravitational acceleration. This led to various models being proposed for lunar formation to account for these unusual concentrations.
Then came Von Braun's statements to Time magazine about three weeks after the first Apollo landing, in which he let slip an important bit of data, namely, that the equigravisphere - the point at which gravitational attraction on an object between the Earth and the Moon was nearly equal - was approximately 43,500 miles from the surface of the Moon. For those who knew the numbers, this was a bombshell, for this meant the Moon was at least acting like an object with more mass than had hitherto been suspected, calling into question previous calculations which had placed it much closer to the surface of the Moon.
It therefore seems reasonable to conclude that NASA is aiming to probe the mysteries of the MASCONS (Mass concentrations) on the Moon with more accuracy than hithertofore, and to plumb the mysteries of the Lunar gravitational anomalies in great detail. Whether they find what they're looking for - the "Grail" - remains to be seen, but the fact that they're looking at all says a great deal.