SOUTHEAST ALASKA — A mountain collapse in Tracy Arm Fjord in Southeast Alaska sent 64 million cubic metres of rock into the water and generated a megatsunami wave reaching almost 500 metres in height, the second tallest ever recorded. The collapse occurred in under a minute during the early hours of the morning, and tourist cruise ships in the area were not caught by the wave because of its timing.

The volume of rock that fell into the fjord was equivalent to 24 Great Pyramids. The largest megatsunami on record remains the 1958 Lituya Bay event in Alaska, which reached over 500 metres.

Megatsunamis occur when a landslide caused by an earthquake or rockfall hits water, and localized waves of this kind usually dissipate quickly. Alaska's steep mountains, narrow fjords and frequent earthquakes leave the region especially vulnerable to megatsunamis.

Dr Bretwood Higman, a geologist, visited Tracy Arm Fjord a few weeks after the event and observed broken trees littering the mountainside and floating in the water, along with large areas of rock stripped of soil and vegetation. "It was a close call." Higman said. "I'm quite terrified that we're not going to be so lucky in the future."

Research published in Science suggests glacier melt driven by climate change is increasing the severity of such collapses. The research team combined field work, seismic and satellite data to reconstruct the sequence of events and measure the wave height.

Dr Stephen Hicks, a geologist, pointed to the growing exposure of visitors to such hazards. "More people are now going to remote areas - often these tourist cruises are going to see the natural beauty of the area to actually learn more about climate change - but they are also dangerous places to be," Hicks said.

The scientists are calling for wider monitoring of hazards in parts of Alaska vulnerable to megatsunamis. Some cruise companies have announced plans to stop sending ships into Tracy Arm due to safety concerns.