A new research has revealed that Arun River, one of the major tributaries of the Koshi River, may be behind Mount Everest’s peculiar growth.

A river piracy event some 89,000 years ago in the Koshi River drainage system might have triggered geological events that have made the tallest mountain in the world about 15 to 50 metre taller, the study, by the researchers at University College London (UCL), says. The Arun River drains the southern expanse of Tibet and the northern slopes of Everest before flowing through a narrow gorge that drops 7 km in elevation over 35 km.

According to the study, the Arun River likely captured another river or water system in Tibet and gained more strength to erode the landmass in the eastern Himalayas, triggering isostatic compensation.

Adam Smith, one of the co-authors of the study, told the BBC that it was a bit like throwing a load of cargo off a ship. “The ship becomes lighter and so floats a little higher,” said Smith. “Similarly, when the crust becomes lighter… it can float a little higher.”

The Himalayas, which consists of the world’s tallest peaks, came into existence after the Indian and the Eurasian plates collided 40 to 50 million years ago. And they have been growing ever since due to the pressure from the collusion.

However, the odd growth of 8,848.86-metre high Mount Everest, or Sagarmatha in Nepali and Chomolungma in Tibetan, has puzzled scientists across the globe.

Recent GPS measurements showed that Everest was growing at a rate of about 0.08 inches (2 millimetres) per year, rather than the expected 0.04 inches (1 millimetre) per year, according to CNN.

And the study published in Nature Geoscience on Monday, September 30, claims to have uncovered a previously unrecognised mechanism of rock uplift active since river capture, that might be responsible for the extra growth.

The study has concluded that Everest partly owes its anomalous elevation to drainage capture and river incision, though tectonic processes governing crustal thickness and rock uplift are clearly the fundamental cause of its elevation. River incision lowers the elevation along the river channel, but the regional area is subject to rock uplift by isostatic rebound, it says, adding that the same effect has been seen in Lhotse and Makalu, the neighbouring peaks.

“The interaction between the erosion of the Arun river and the upward pressure of the Earth’s mantle gives Mount Everest a boost, pushing it up higher than it would otherwise be,” Dr Xu Han of China University of Geosciences, who was the lead author in the study, told the BBC.