Researchers have published an investigation into the mysteries behind an enormous “hidden” earthquake that caused a global tsunami last year. In a study published in Geophysical Research Letters on February 8, scientists said that a total of five earthquakes, including one massive, “hidden” tremor, created the tsunami after striking close to the South Sandwich Islands on August 12, 2021, in the remote Southern Ocean off the coast of the continent of South America.

The study showed that tsunami-causing earthquakes such as the August 12 incident had complex patterns that could confound how scientists measure major tectonic activity. LiveScience reported that in August 2021, an enormous tsunami rippled out into the North Atlantic, Pacific and Indian oceans.

It was the first time a tsunami had been recorded in three different oceans since the catastrophic 2004 Indian Ocean earthquake; at the time, scientists thought it was caused by a 7.5-magnitude earthquake detected near the South Sandwich Islands (a British Overseas Territory in the southern Atlantic Ocean).

But not everything was as it seemed. Scientists were baffled to find that the supposed epicenter of the earthquake was 30 miles (47 km) below the ocean floor, which is far too deep to cause a tsunami, and that the tectonic plate rupture that spawned it was nearly 250 miles (400 km) long — that kind of rupture should have caused a much larger earthquake.

Now, a new study published Feb. 8 in the journal Geophysical Research Letters, has revealed that the earthquake was actually a sequence of five sub-quakes, separated in time by mere minutes. And the third of these mini-quakes — a shallower, “invisible” earthquake hidden in the data and missed by monitoring systems at the time — was an 8.2-magnitude quake responsible for the tsunami.