Apollo 12 carried the first lunar module ascent stage to be intentionally crashed into the Moon.
After Charles Conrad and Alan Bean lifted off from the lunar surface in Intrepid, the ascent stage was no longer needed. Mission controllers commanded it to impact the Moon on 20 November 1969, producing a signal detected by seismometers deployed by the astronauts.
That impact helped scientists study how seismic waves traveled through the lunar interior. Apollo 12's landing near Surveyor 3 also made the mission valuable for examining how long-term exposure to the lunar environment affected hardware.
The event is sometimes confused with impacts from spent Saturn rocket stages or later lunar-module ascent stages. Apollo 11's ascent stage was also eventually sent into a lunar impact trajectory, but Apollo 12's deliberate impact was the first in the Apollo landing program.