A mystery on the Aran Islands off the Irish West Coast has finally been solved by one of the world’s top geologists.

Something had picked up massive boulders off a beach and propelled them over high cliffs to a flat landscape beyond. The largest of these boulders weighed about 78 tons, and they now lie some 40 feet above the Atlantic Ocean. Smaller boulders, weighing about 4 tons each, lie more than 820 feet inland.

"The local people say that these rocks are moving," said geologist Ronadh Cox, a professor of geosciences and chairwoman of the maritime studies program at Williams College in Massachusetts.

The mystery of how they got there has finally been solved.

The most likely culprit, a tsunami, or at least very large waves. A tsunami hasn't hit the country since 1755 when a magnitude 8.7 earthquake in Portugal sent tidal waves across the ocean to Ireland.

By dating the rocks, they proved that some arrived on land thrown up from the ocean 2,000 years ago, but others arrived less than 50 years ago, a key piece of evidence that leaves powerful storm waves holding the smoking gun, Cox said.

A local man provided much-needed evidence when he recalled a ferocious storm in 1991 that deposited massive boulders from the seabed up a cliff and hundreds of feet away. 

Cox hunted through Irish government data and found that in the winter of 1991, a particularly ferocious storm did indeed hit the area. 

The mystery was solved and the ocean’s power is truly astonishing. "The waves can just climb these cliffs in amazing ways," she said.

Read more: 170-year-old mystery of famed Irish Arctic explorer solved

* Originally published in 2013.