Immense ice sheets have grown and shrunk many times (Image: Ashley Cooper/SpecialistStock/SplashdownDirect/Rex Features) Ice age timeline

BACK in 1993, a boy playing football near Nanjing, China, suddenly fell through the ground. He had inadvertently found a new cave, later named Hulu, which has turned out to be a scientific treasure chest. Besides two Homo erectus skeletons, it contains stalagmites that have helped solve one of the greatest mysteries in climate science: why the ice ages came and went when they did.

For more than 2 million years, Earth’s climate has been oscillating wildly. Immense ice sheets slowly advance across northern lands, then suddenly melt away to leave the planet basking in a relatively brief period of warmth before the ice creeps back again. Climate scientists have long suspected that these glacial cycles are triggered by changes in our planet’s orbit. Yet while this theory has had many successes, it fails to explain one critical fact: why the ice ages end every 100,000 years or so. “It’s a big problem,” says Larry Edwards of the University of Minnesota in Minneapolis.

Edwards is part of a group of researchers who may finally have the answer, thanks to Hulu and other nearby caves. If their conclusions are right, then the greatest ice sheets of the past were remarkably vulnerable, melting away when there was just a glimmer of extra sunlight. But what have stalagmites in China got to do with the vast ice sheets that covered much of Europe and Siberia, and North America?

By the middle of the 19th century, it was clear that there had once been a “Great Ice Age” …