 Gravitational anomalies slightly distorting the orbits of some Lunar Orbiters led to the discovery of mass concentrations (dubbed mascons) beneath the lunar surface caused by large impacting bodies at some remote time in the past.These anomalies are of sufficient magnitude to cause a lunar orbit to change significantly over the course of several days.They can cause a plumb bob to hang about a third of a degree off vertical, pointing toward the mascon, and increase the force of gravity by one-half percent.