Long before New York City was the Big Apple, or even New Amsterdam, a giant tsunami crashed ashore.
It was 2,300 years ago. The Palisades that frame the Hudson River were whisper-quiet, the sandy beaches of Long Island and New Jersey empty, and Manhattan was still just an unbroken sylvan carpet.
Then came the mammoth wave, roaring into the serenity. No one knows for sure what caused it, but new clues found in the Hudson's silt suggest an asteroid 100 meters (330 feet) in diameter slammed into the Atlantic Ocean nearby.
"But the main thing that closes the deal is that we looked in the spherules and found nano-diamonds," said Dallas Abbott of Columbia University, a co-author on the work. "These have only been found in impact ejecta or in meteorites."
The team found grains of several shocked minerals in the sediments as well, but the discovery remains controversial.
"To get a wave 2.5 meters high that far up the Hudson, you need a wave 20 meters high at Manhattan," said Steven Ward of the University of California, Santa Cruz. "It would've gone several hundred meters inland on Long Island; you should see evidence of this thing all over the place."
Even worse, telling the difference between sediments washed up in a tsunami and those left by a strong storm can be incredibly difficult.
So far the team has only found impact ejecta in deposits in the Hudson, with some as far as 50 kilometers (31 miles) upriver from the mouth. But they have taken samples of suspicious-looking sediments along the coasts of New Jersey and Long Island as well, and hope to find more of the same strange minerals pointing to an impact origin.
"We've had strong storms in New York's history that haven't made deposits anything like this," Cagen said. "We don't know how big it was, but it would have been more than a splash against Manhattan; the city would have been devastated."
Cagen is convinced her team's work proves an impact caused the tsunami but admits they'll need to find the smoking gun — a crater, probably buried in the continental shelf off New Jersey — to convince skeptics.
"We're making the pretty outrageous claim that not only did a tsunami hit the New York metropolitan area 2,300 years ago, but it was caused by an asteroid impact for which we can't find a crater," she said.