If you happened to be present in the archipelago country of Cape Verde some 73,000 years ago, researchers say you likely would have witnessed a mega-tsunami more than 800 feet tall.

The island chain off the coast of Africa is home to Fogo volcano, one of the world's largest and most active island volcanoes. Viewed from above, the eastern flank of the island carries a massive scar from an ancient landslide. While earlier studies theorized this collapse happened over a period of smaller events, scientists now believe it occurred all at once — with the more than 40 square miles of earth that plowed into the sea generating a wave taller than a 65-story building.

Fogo TsunamiFogo volcano, as seen in this NASA photo, suffered a dramatic landslide event on its eastern slope some 73,000 years ago. (Photo: NASA)

"Our point is that flank collapses can happen extremely fast and catastrophically, and therefore are capable of triggering giant tsunamis," said lead author Ricardo Ramalho.

Ramalho first discovered evidence of a monster wave after researching the origin of gigantic marine-type boulders on Santiago Island. The site, 34 miles away from Fogo, contains massive rocks (some weighing more than 770 tons) as far as 2,000 feet inland and nearly 650 feet above sea level.

Fogo TsunamiA researcher examines one of the massive boulders on Santiago Island thrown inland by a mega-tsunami 73,000 years ago. (Photo: Ricardo Ramalho/Columbia University)

"At first, we were quite puzzled by what we found in the field — why were such large boulders stranded on the landscape?" Ramalho told LiveScience. "We got really excited when we realized that the only way to explain the origin of those boulders was with a massive tsunami impact."

Researchers estimate that when the Fogo mega-tsunami struck Santiago Island, it was likely 557-feet-tall and powerful enough to rip boulders off the coastline and throw them inland. By comparing the age of the collapse on Fogo to the age of the boulders strewn on Santigao's plateau, the scientists were able to link the two events.

Evidence of past volcanic landslides, and their catastrophic results, should comes as warnings to other island nations in active volcanic regions. As this recent study further proves, it's not just earthquakes that can trigger monster waves.

"Our work shows that we need to be vigilant and that we should not underestimate the threat posed by flank collapses and the tsunamis they trigger, but our work doesn't imply that a flank [collapse] is about to happen on Fogo or anywhere else," Ramalho added to LiveScience. "Each volcano needs to be monitored and analyzed separately and in detail."