A new map of Mars has revealed mysterious structures hiding beneath the sediment layers of a lost ocean.

The researchers detected about 20 features scattered around the planet's north polar cap that are significantly denser than their surroundings. 

The structures vary in shape and size, with one resembling the shape of a dog, leaving the team puzzled because they do not know exactly what the formations are or where they came from. But they do have some theories.

One idea is that the structures were compacted by ancient meteor strikes, or  formed by volcanic activity, but more research is needed to get to the bottom of these underground anomalies. 

The team of researchers from Denmark's TU Delft and Utrecht University presented their findings at the Europlanetary Science Conference in Berlin this week.

The team used tiny deviations in satellite orbits to create a picture of Mars' gravitational field, or the region of space around a planet where its gravitational force can be felt. 

They did this to look for clues about how mass is distributed throughout the planet's subsurface. 

The team then combined these observations with data on the thickness and flexibility of the Mars' crust, as well as the dynamics of the planet's mantle and deep interior. 

These observations were collected by NASA's InSIGHT lander, which completed its study of Mars' crust, mantle and core in 2022.

This allowed the researchers to create a global density map of Mars that revealed the existence of 20 previously unknown underground structures scattered around the planet's north polar cap.

The structures are about 19 to 25 pounds per cubic foot denser than their surroundings, and vary in shape and size. 

Additionally, they are covered by a thick, smooth layer of sediment that may have once been a seabed. 

Billions of years ago, Mars was not the desert planet we know today. It was once covered in oceans and rivers, but water dried up in an extreme climactic shift. 

Now, the only evidence of these bodies of water lies in Mars' geologic record - like this sediment layer. 

As for the structures that lie beneath, 'there seems to be no trace of them at the surface,' lead author Bart Root, an assistant professor at TU Delft, said. 

'However, through gravity data, we have a tantalizing glimpse into the older history of the northern hemisphere of Mars.' 

This isn't the first time that scientists have uncovered oddities beneath the surface of Mars. 

In December 2023, China's Zhurong found large honeycomb-shaped crevasses buried dozens of meters beneath Mars' equator that likely formed when drastic temperature dips contract and fracture the ground.

But Root and his team are having a harder time figuring out what these most recently structures are, and where they came from. Right now, they have two main theories. 

Either the structures were compacted by ancient impact events, like meteor strikes, or they were formed by some kind of volcanic activity. This latter idea challenges scientists' longstanding view of Mars as a geologically inactive planet. 

But while studies have shown that Mars does not have as much geological activity as Earth, a growing body of evidence suggests that it isn't completely 'dead.'

Root's study adds to this evidence not just by discovering structures that may be volcanically formed, but through a totally separate finding. 

In addition to finding the mysterious structures, the team's analysis revealed that Mars' mantle may still host active geological processes that could be feeding Olympus Mons, the largest known volcano in the solar system.

Olympus Mons is located in the Tharsis Montes region near Mars' equator. Scientists estimate that it hasn't erupted for 25 million years.

The subterranean geology of the Tharsis region is incredibly dense, but Root and his team detected a much lighter mass lying 700 miles beneath the surface. 

He believes this mass is an enormous plume of magma that stretches 1,000 miles across in Mars' mantle. What's more, this plume might be in the process of bubbling to the surface.

'This means we need to rethink how we understand the support for the Olympus Mons volcano and its surroundings,' Root said. 

'It shows that Mars might still have active movements happening inside it, affecting and possibly making new volcanic features on the surface.'