The strange skull appeared soon after the Japanese invaded northeast China in the early 1930s. A team of locals was raising a bridge near Harbin, a city in China’s northernmost province, when one of the workers stumbled on a surprise in the river mud. The nearly complete human skull had an elongated cranium from which a heavy brow bone protruded, shading the gaping squares that once housed eyes.

And then there was the skull’s unusual size: "It's enormous," says paleoanthropologist Chris Stringer of London's Natural History Museum.

Perhaps aware of the magnitude of the find, the man secreted the skull away in an abandoned well. Now, nearly 90 years later, a study published in the journal The Innovation