After billions of years of solitary travel through cold, dark outer space, comet 67P/Churyumov-Gerasimenko has recently yielded many secrets, often surprising ones, to the prodding of Europe's Rosetta spacecraft.

As the probe drew closer and got a better view, ground teams were amazed to discover their target had a weird rubber-ducky shape, an ultra-dark, rough and carbon-rich surface with boulders and dunes, and gave off the most awful chemical "smell".

On Wednesday, researchers said they have uncovered yet another of the comet's long-guarded secrets: it is dotted with deep, cylindrical pits likely created through a process similar to sinkholes on Earth.

"These strange, circular pits are just as deep as they are wide. Rosetta can peer right into them," said Dennis Bodewits of the University of Maryland, who co-authored a paper published in the journal Nature.

"We propose that they are sinkholes, formed by a surface collapse process very similar to the way sinkholes form here on Earth" -- where subsurface erosion creates caverns, the ceilings of which eventually collapse under their own weight.

"We already have a library of information to help us understand how this process works, which allows us to use these pits to study what lies under the comet's surface," Bodewits said in a statement.

The European Space Agency launched Rosetta in March 2004 on a 10-year journey to meet up with 67P.

After travelling some six billion kilometres (3.8 billion miles), the spacecraft caught its first glimpse of the comet in March 2014, from a distance of about five million km -- when it was just a small dot in the sky.

Five months later, Rosetta became the first spacecraft to enter into orbit around a comet, and in November made further history by landing the Philae robot on 67P's surface.

Streaking towards the Sun at a speed of 32.5 kilometres (20 miles) per second, the comet will reach perihelion -- its closest point to our star -- on August 13, before looping back out into outer space with Rosetta and Philae still in tow.

Comets are deemed to be frozen balls of dust, ice and gas left over from the Solar System's formation some 4.6 billion years ago, and scientists hope that unravelling their makeup may provide insights into Earth's own creation.

One theory is that comets smashed into our infant planet, providing it with precious water and the chemical building blocks for life.

- Sub-surface heat source? -

Some of Rosetta's surprise discoveries so far include that "67P" has much less surface water ice than expected, and appears to have no magnetic field.

Now, thanks to pictures taken by Rosetta's Osiris camera, we also know it features pits, typically about 200 metres (656 feet) in diameter and some 180 m deep. Rosetta has spotted 18 of them in the comet's northern hemisphere, according to the paper.

Jets of gas and dust streamed from the deeper pits, said the study team, but not the shallower ones.

Measurements ruled out "explosive events" as the source of the holes, said the researchers, making the sinkhole explanation more likely.

"A source of heat beneath the comet's surface causes ices (primarily water, carbon monoxide and carbon dioxide to sublimate (from a solid to a gaseous state)," postulates the paper.

The voids created by the loss of these ice chunks eventually grow large enough that their ceilings collapse under their own weight, giving rise to the deep steep-sided circular pits seen on the surface of comet 67P/Churyumov-Gerasimenko."

The deepest pits are thought to be the "youngest", filling over time with dust and ice chunks that erode their walls and form the shallower holes also observed, said the team.