A rover belonging to China’s space program, the China National Space Administration (CNSA), found a goo-like substance on the moon. According to People’s Daily, a popular newspaper in China, the Yutu-2 rover is part of China’s Chang’e-4 mission. This mission is the first to have ever soft-landed on the far side of the moon, and was accomplished in January of 2019.

According to Space, the Chang’e-4 team discovered the substance on lunar day 8 of the rover’s exploration, or July 28, 2019. Every day, the Yutu-2 powered down for a mid-day “nap” in order to keep it from overheating. As operators prepared to shut the rover down, they noticed the goo in the images from the rover’s main camera.

After the rover had its nap, operators sent it to check out the goo. Scientists on the mission described it as “gel-like” and said it had an “unusual color.” Some outside researchers theorize that the substance might be melted glass created by a meteor strike.

Scientists still haven’t identified the exact nature of the mysterious space goo, but are continuously working to get some answers.