We might’ve been to the moon, but our celestial neighbor still holds many secrets. One of them, however, has just been unearthed — err, unmooned? — by a team of international researchers. For the first time, they’ve proved the existence of a cave on the moon.

The cave is located in the moon‘s Sea of Tranquility, some 250 miles (400 kilometers) from the Apollo 11 landing site, and it’s 130 feet (40 meters) wide and 10s of yards long. “These caves have been theorized for over 50 years, but it is the first time ever that we have demonstrated their existence,” Lorenzo Bruzzone, a professor at Italy’s University of Trento, said in a statement.