The more information astronomers gain about comets, the more mystifying and wonderful the ancient icy dust balls prove to be. And these days, they're learning a lot. The European Space Agency's Rosetta orbiter, traveling around comet 67P/Churyumov–Gerasimenko halfway between the orbits of Earth and Mars, has been collecting data ever since it arrived at its destination last August. And now, for the first time, researchers have used some of its images to link the comet's activity—the tendency for its jets to spew ice, dust, and gas into the cosmos—to features on the space rock's surface.

Those features are sinkholes, say Jean-Baptiste Vincent and colleagues today in Nature, and off-loading jet sites can form within those blotches on the comet's face. Perhaps even more exciting, close-up views of these pits are giving the first glimpse at what lies beneath the comet's surface.

What's there? Dinosaur eggs. Dino eggs and the first clues to a fresh mystery. "In the walls of these pits there are strange things, though we don't fully understand what they are" says Vincent, a planetary scientist at the Max Planck Institute. "We see lots of fractures and features that look like pebbles—some people call them 'dinosaur eggs.' They look like the primordial pieces that make up the comets to begin with." Scientists don't know how they're produced, he says, but they're only seen in the comet's pits.

That said, the sinkholes themselves are extremely unexpected. "I don't think anyone predicted anything like this," says Paul Weissman, a senior research scientist who has been at NASA's Jet Propulsion Lab for 40 years and authored an essay commenting on the work in today's Nature. "You can predict from known physics certain things, but the world is more complex than our knowledge of how to do the physics is right now."

Rosetta didn't set out looking for sinkholes, but they were hard to ignore once it (and Philae!) arrived. The space orbiter's team noticed a particularly large feature (about 650 feet across and just as deep) when they began to map the surface. "It was something puzzling because we had never observed these before," says Vincent. It was also tough not to notice that the shadows of these pits weren't completely black—they were releasing a ton of material.

The icy cometary material was sublimating—transitioning directly from solid to gas (there isn't enough pressure on comets to form liquids). And the gas wasn't coming from the floors of the pits; it was emerging from the walls. Likely, that's because solids that can't sublimate fall to the floor, covering up the bottom. This process may, in time, affect the overall morphology of the comet.

Less exciting, but located around the same area as the pits, Rosetta's team also found large, flat-bottomed basins. "These can't be impact craters," says Weissman, "because there are too many big ones." Impact craters would leave a predictable size distribution. Similarly large, shallow basins have been spotted before, on other comets, but their formation history was anybody's guess.

Vincent and his colleagues suspect that the cometary sinkholes form in much the same way as terrestrial sinkholes—material is eaten away from within a porous interior, creating a hollow underground cavity. When the ceiling cannot not support its own weight, material falls in, creating a circular, deep hole.

If this is how 67P's sinkholes form, it jibes nicely with Weissman's theory, first published in 1986, that comets are essentially rubble heaps, not smooth solid masses. "But we don't quite know yet," he says. He's seen other, conflicting evidence, suggesting that the comet's interior might be smooth after all.

"We basically have the story of the eight blind men with the elephant and everyone thinks it's different," he says. "One of my jobs on the mission is to figure out how the eight pieces of information fit together."

The coming weeks will be action-packed for astronomers with a penchant for comets—as 67P approaches the Sun and heats up, it grows more active. The jets spew more prolifically. As they do, Rosetta's chemical sensing instruments may be able to spot rarer species of compounds.

These could possibly include amino acids, which have been spotted before on meteorites. If seen bursting from cometary jets, these molecules—crucial components in the formation of all known life—will make 67P not only more mysterious and glorious, but a little more familiar as well.