A year ago, China’s Yutu-2 rover found a mysterious, green, gel-like substance on the surface of the moon.

Chinese scientists thought it might be melted rock formed after some sort of impact, and they had the rover take two additional passes at the goo in order to collect more data; at first, according to cNet, the area was covered in shadow, making data collection impossible.

It is melted rock formed after some sort of impact.

The rock, or to be more specific, the “impact melt breccia,” is described there as “dark greenish and glistening” and is said to be similar to the samples of impact melt breccia collected by astronauts on the Apollo missions.

Breccia, according to the Planetary Science Institute, is a mixture of different types of fragmented rocks, held together by something; with an impact melt breccia, that something is “impact melt,” which occurs when an impact causes rock to melt.