An unusual rock buried beneath the far side of the moon is offering fresh evidence in a scientific debate that has persisted for decades. Researchers found that the subterranean formation is much denser and more strongly magnetized than the material surrounding it.

The structure may be an ancient volcanic complex formed when magma rose from the moon’s interior but solidified before reaching the surface. The study was published September 23 in the journal Science Advances.

The buried body measures about 60 kilometers, or 37 miles, across and extends roughly 9 kilometers, or 5.6 miles, deep. Its characteristics suggest the moon possessed a magnetic field when the rock formed about 4.2 billion years ago.

According to the study authors, that ancient field was likely between a fifth and a third the strength of Earth’s current magnetic field. The discovery provides evidence from a region called Dewar, which cannot be seen from Earth.

Earth’s magnetic field creates the magnetosphere, a vast region around the planet that helps block harmful solar wind, cosmic rays and radiation. Without that protection, Earth’s atmosphere could be stripped away, making life as it is known far less likely.

The constant movement of molten metal in Earth’s outer core generates electrical currents that sustain its magnetic field through the dynamo effect. The moon may also have possessed a magnetic field powered by its core, although that activity ended long ago.

NASA has said the lunar field weakened significantly about 3.2 billion years ago as the moon’s interior cooled. Without strong magnetic protection, solar radiation tore apart the lunar atmosphere.

“The debate stretches back to the 1970s, when the Apollo missions first returned lunar rocks that showed signs of having recorded a magnetic field,” said study coauthor Anna Mittelholz, a lecturer at the Swiss Federal Institute of Technology Zurich, or ETH Zurich.

Experts broadly believed the lunar dynamo operated from about 4.25 billion to 3.5 billion years ago, Mittelholz said. However, analyses conducted during the past decade found no magnetic signal in some Apollo samples from parts of that period, prompting some researchers to question whether an early dynamo existed.

The latest study avoids relying on Apollo samples. Instead, the researchers examined orbital information collected by NASA probes, including Lunar Prospector, launched in 1998, Kaguya, launched in 2007, and GRAIL, launched in 2011.

Orbital observations offer an advantage because Apollo samples were removed from their geological setting more than 50 years ago. Mittelholz said their magnetic records can be disrupted by heating, shock, handling or laboratory storage, potentially causing similar samples to produce different answers.

Rather than studying magnetic measurements alone, the team combined magnetic and gravity data in one model. Gravity measurements revealed the density of the buried rock, while magnetic information showed how strongly it was magnetized, allowing the researchers to identify a physical structure rather than an isolated signal.

The researchers could not determine how long the lunar dynamo remained active. They also said it remains unclear how the moon’s small core could have generated a field potentially as strong as earlier studies have suggested, but they believe the findings redirect the debate toward how the dynamo operated.

The study also examined lunar swirls, bright patches on the moon’s surface that are often associated with magnetic anomalies. Lead author Xi Yang, a doctoral student in ETH Zurich’s department of Earth and planetary sciences, said the leading explanation is that these anomalies deflect solar wind and protect the surface from weathering.

Yang said the preliminary observation could eventually help identify lunar areas with greater protection from harmful solar wind, making them valuable for future missions. Claire Nichols, an associate professor of geology of planetary processes at England’s University of Oxford who was not involved in the study, said more work is needed to determine the dynamo’s duration and variability.

Isaac Narrett, lead author of an unrelated 2025 study, said the research adds to evidence that the moon had a stronger magnetic field more than 4 billion years ago. Narrett said its use of orbital data also creates opportunities to study planetary evolution through gravity and magnetic data from the moon, Mercury and Mars.