Researchers have found evidence of a huge earthquake building below Bangladesh, the world's most densely populated country.

With so little historical data from the region, geologists can't predict when the quake will occur, but they say it's likely to be between a magnitude 8.2 and 9 when it hits, and at least 140 million people in the region could be affected.

"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," said lead researcher Michael Steckler from Columbia University. "We can't say it's imminent or another 500 years. But we can definitely see it building."

The newly identified threat is the result of 13 years of surface monitoring in the region, which is where the Indian and Sunda tectonic plates meet. Although scientists knew that there was tension between the plates before, they had assumed they were just sliding alongside each other horizontally - which can cause minor earthquakes.

But after using ground and satellite GPS monitors to measure surface activity between 2003 and 2013, an international team of researchers has shown that Bangladesh is actually sitting on a huge subduction zone - which means the Indian plate is actually thrusting underneath the Sunda plate at a rate of 17 millimetres (two-thirds of an inch) a year.

All of the worst earthquakes on record have happened along subduction zones, including the Indian Ocean quake that killed around 230,000 people in 2004, and the 2011 Japanese tsunami-triggering quake.

But this is the first subduction zone discovered underneath land - and the data show that a 250-km area running right underneath the capital city of Dhaka is locked and has been loading up with stress for at least 400 years without any release.

"Some of us have long suspected this hazard, but we didn't have the data and a model," said Steckler. "Now we have the data and a model, and we can estimate the size."

Those models predict that if all the pressure of the subduction zone was released at once - the worst-case scenario - it could cause the land to jump horizontally between 5 and 30 metres (18 and 100 feet).

And seeing as the subduction zone is located under the world's largest river delta - a muddy region where the Ganges and Brahmaputra rivers meet the sea - the researchers that the quake could turn the whole area into quicksand.

The research has been published in Nature Geoscience.

"The great rivers - [16 kilometres] (10 miles) across in places - could jump their banks and switch course, drowning everything in the way; there is in fact evidence that such switches have happened in previous centuries," the researchers explain.

Of course, there's no need to panic just yet - we only have 10 years of reliable data analysed, which isn't long geologically, so more research is needed to verify these models and replicate their predictions.

And these are all worst-case scenarios - in the ideal situation, only part of the subduction zone will release at a time, causing smaller, more manageable quakes.

But the results of the modelling highlight how important it is for the region to prepare itself - before 1993, Bangladesh had no building codes, and one of the researchers, Syed Humayun Akhter, a geologist at Dhaka University, says that the country is greatly unprepared for a quake of this size.

"Bangladesh is overpopulated everywhere," he said. "All the natural gas fields, heavy industries, and electric power plants are located close to potential earthquakes, and they are likely to be destroyed. In Dhaka, the catastrophic picture will be beyond our imagination, and could even lead to abandonment of the city."

Roger Bilham, a geophysicist at the University of Colorado who wasn't involved in the research, said that the "data are unassailable, the interpretation is sound … the Indian subcontinent is effectively being pushed into a tight corner", but agreed that more research was needed.

Scientists in the region are now trying to do just that, with a team from New Mexico State University planning to deploy 70 seismometers across Myanmar next year.

Let's hope with more data we'll get a clearer picture of what's going on below one of the most populated regions on the planet, and come up with a plan to keep the population safe.