Before the twentieth century, astronomers believed volcanoes formed the Moon's craters instead of meteorite impacts. Now we know better, but those earlier astronomers might not have been completely wrong - at least when it comes to craters like this one.
We know that the Moon was once volcanic billions of years ago. Indeed, all the blackened, darker regions on the moon are huge plains of basalt left over from primordial lava flows. Astronauts on the Apollo 17 mission discovered orange glass that conclusively proved the Moon had once been volcanic, and there are also narrow lunar gullies called rilles that could only have been formed by flowing lava.
But the task of actually finding an extinct lunar volcano is much more difficult, because it's pretty much the definition of a needle in a haystack, with a few volcano craters scattered among countless impact craters. That's what's so exciting about this new crater spotted by the Lunar Reconnaissance Orbiter in the Lacus Mortis ("Lake of Death" in Latin, because astronomers are nothing if not cheery).
What makes this different from other craters? It's the gentle slopes of both the inner and outer walls that set it apart, which are features that are highly unusual for an impact crater but fit a volcano perfectly. The battered, weathered regions around the crater are also characteristic of areas once flooded with hot lava, while the area surrounding most impact craters are surrounded tends to be undisturbed.
Although the lunar domes are known examples of shield volcanoes on the moon, this crater would be the first known example of the mountain-like stratovolcano, which is the iconic type most people picture when they think of volcanoes. The crater is located near another one with similar features, which might mean this entire area was once a hotbed of volcanic activity. Unfortunately, there's no way to definitively prove what type of crater it is unless we send a geologist - be it human or robotic - to the Moon to get a closer look.