Statio Tranquillitatis

The retroreflector array left here is still in use. It is a panel of 100 fused silica corner cubes that returns a laser pulse to its source regardless of the angle it arrives at, and observatories have been timing pulses off it since 1969. The round trip is measured to a few millimetres.

Fifty years of that data give the Moon's recession from Earth as 3.8 cm a year, confirm the lunar core is liquid from how the Moon wobbles, and test general relativity against alternatives by looking for a deviation in the orbit. It is the only Apollo experiment still returning results.