The Earth has a mysterious 'pulse' which causes regular mass extinctions, major volcanic eruptions and strips the ocean of its oxygen, according to a new study.
And no one knows what's causing the "intermittently catastrophic" changes.
"Many geologists believe that geological events are random over time," said geologist Michael Rampino of New York University. "But our study provides statistical evidence for a common cycle, suggesting that these geologic events are correlated and not random."
His team looked at 89 events of global significance which could be dated over the last 260 million years, including sea level changes and "pulses of continental flood-basalt volcanism" - when large parts of the ocean floor are covered in lava due to massive deep-sea eruptions.
They found the events were "grouped in peaks or pulses" around 27.5 million years apart. That figure lined up neatly with previous research Dr Rampino had conducted into mass extinction events, which also happen about every 27.5 million years.
It's not clear what could be causing it. One possibility is it's driven by "thermal and geophysical disturbances in the inner Earth", or links between tectonic plates and the climate.
But another is that the Earth passes through the plane of the Milky Way galaxy every 32 million years or thereabouts, and it's hypothesised that this could be to blame.
"Encounters with concentrations of disk-dark matter might trigger comet showers", the study suggests. Our solar system is believed to be surrounded by a faraway cloud of icy objects called the Oort Cloud, which can be dislodged by major gravitational sources, sent flying towards us as comets.
One such event happened 66 million years ago - best-known for the extinction of the dinosaurs, the Cretaceous-Paleogene extinction event killed off three-quarters of all species. It's widely believed now to have been caused by something slamming into the Earth - perhaps from the Oort Cloud - but there are competing theories that suggest a more terrestrial cause, or that there were multiple causes.
Dr Rampino says despite the differences in the two cycles, they might actually be one and the same once you allow for irregularities in the data and the difficulty of accurately timing things that happened even before dinosaurs roamed the Earth.
"Whatever the origins of these cyclical episodes, our findings support the case for a largely periodic, coordinated, and intermittently catastrophic geologic record, which is a departure from the views held by many geologists."
The last such 'pulse' happened about 7 million years ago, suggesting we're in the clear for at least the next 20 million years.
The latest research was published in the journal Geoscience Frontiers.