In 1969, a giant earthquake off the coast of Portugal kicked up a tsunami that killed over a dozen people. Some 200 years prior, an even larger earthquake hit the same area, killing around 100,000 people and destroying the city of Lisbon.

Two earthquakes in the same spot over a couple hundred years is not cause for alarm. But what puzzled seismologists about these tremors was that they began in relatively flat beds of the ocean — away from any faults or cracks in the Earth’s crust where tectonic plates slip past each other, releasing energy and causing earthquakes.

So what’s causing the rumbles under a seemingly quiet area?

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