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Bangladesh and its neighbourhood including India may be hit by a huge earthquake - although not imminent - that could reach a magnitude of nine, researchers have warned.
The scientists said they have new evidence of increasing strain building beneath Bangladesh where two tectonic plates underlie the vast delta of the Ganges and Brahmaputra rivers.
They estimate that at least 140 million people in the region could be affected if the boundary ruptures.
The researchers are not forecasting an imminent great earthquake but said it was an "under-appreciated hazard".
"Some of us have long suspected this hazard but we didn't have the data and a model," said lead author Michael Steckler, geophysicist at Columbia University's Lamont-Doherty Earth Observatory in the US.
"Now we have the data and a model and we can estimate the size," he noted.
The destruction could come not only from the direct result of shaking but changes in the courses of the Ganges and Brahmaputra rivers and in the level of land already perilously close to sea level.
The newly identified threat, described in the journal Nature Geoscience, is a subduction zone where one section of the earth's crust, or a tectonic plate, is slowly thrusting under another.
Scientists knew of the plate boundary in and around Bangladesh but many assumed it to be sliding only horizontally near the surface, where it sometimes causes fairly large but less damaging earthquakes in areas that are not as densely populated.
However, the authors of the new research said movements on the surface over the past decade showed subduction was taking place below and a part of the plate juncture was locked and loading up with stress.
Steckler said strain between the plates had been building for at least 400 years.
When an inevitable release comes, the shaking was likely to be larger than 8.2 on the Richter scale and could reach a magnitude of nine, similar to the largest known modern quakes, Steckler warned.
"We don't know how long it will take to build up steam because we don't know how long it was since the last one," he said.
"We can't say it's imminent or another 500 years. But we can definitely see it building," Steckler said.
James Ni, a seismologist at the New Mexico State University, said he and colleagues hoped to deploy 70 seismometers across Myanmar in 2017 to get a better image of the apparently subducting slab.
"We don't have a good idea of its geometry, we don't know how far it goes down," Ni said.
He said if the study authors were right and the slab was building strain, a quake would probably turn urban areas in eastern India "into ruins", and its impact was likely to extend into Myanmar and beyond.
