A dense mass under the moon’s south pole has been confirmed by gravitational testing, but scientists are still not sure what it is.

The site is the location of the solar system’s largest-known crater, and details of the analysis are provided in a Popular Science report.

The moon’s south pole — and the moon overall — holds secrets that are largely unexplored. The south pole constantly faces away from the Earth, and there was very little known about it before 2011 and 2012, when two lunar orbiters spent a year collecting gravity field and geologic data.