On April 14th, 1970, a new crater was carved into the surface of the Moon:
How do we know it's new? Because we made it.
That's the impact scar of the third stage of the Saturn V rocket (technically designated S-IVB) that carried Apollo 13 to -- but sadly, not on -- the Moon. Earlier missions had placed seismic instruments on the lunar surface to measure if the Moon had any activity. They found it did, and in fact several moonquakes were big enough that had you been standing there, you would have felt them quite strongly (and probably been knocked on your spacesuit's backside).
The S-IVB upper stage accelerated the astronauts to the Moon from Earth orbit. Once that was done, they had one final mission: in Apollos 13 - 17 the stages were aimed at the Moon itself, and impacted a few days later. The impacts were detected by the seismometers and could be used to determine how seismic waves travel through the lunar surface, a trick that's been used on Earth for a long time. This information can be used to figure out what the lunar subsurface structure is like.
The crater image above is from the Lunar Reconnaissance Orbiter, and shows the Apollo 13 booster impact. The crater itself is a few dozen meters across, and the material ejected forms a blanket around it for many meters more. The bright material indicates this is a fresh crater; note how gray the more distant undisturbed material around the crater is.
The impact site looks obvious in that picture, doesn't it? But try finding it in the original full-resolution image returned from LRO and see if you can locate it, then! I found it relatively quickly starting at the top, and was shocked at how far I could trace the rays -- the linear ejected debris features around the crater -- from the impact site. One of them is clearly about a kilometer long... that's over half a mile! Those rays are from plumes of material ejected from the impact site, a common feature. They also indicate the crater's youth: over time, cosmic rays, the solar wind, and even thermal stress from the Moon's day/night cycle slowly erase the rays. Any crater with such extensive rays has to be young.
Some of the other S-IVB impact sites have been identified; the LRO blog has an image of the Apollo 14 S-IVB crater, for example. Knowing where these impact sites are helps scientists understand the Moon better, since it a more precise location means the data from the old Apollo missions can be interpreted more clearly. I wonder if future colonists may visit those sites the way we do Plymouth Rock, or Jamestown, or other early exploration and colony sites on Earth?
Credit: NASA, NASA/GSFC/Arizona State University