A region of northern India has been called the world's worst place for altitude sickness, and scientists say nobody knows exactly why.

Ladakh, a high-altitude desert on India's northern border with China and Tibet, has become a popular destination for adventure travelers in the last decade who come to experience its remote culture and extraordinary beauty, Britain's The Independent reported Monday.

In a large study of visitors to mountainous regions around the world assessed for their vulnerability to altitude sickness, Ladakh was found to be the biggest threat.

"When adjusted for all other risk factors, especially rate of ascent, one location -- Ladakh -- remained associated with a higher risk of severe high-altitude illness," study leader Jean-Paul Richalet, professor of physiology at the University of Paris North, said.

Scientists said they were unable to discover why the particular area should present such a threat.

"No clear explanation, linked to the climate or the difficulty of the terrain is available, although many informal reports mention the higher risk of this location," Richalat said.

In the study, researchers assessed more than 1,300 people who were planning excursions to mountainous areas that included at least three days above 13,000 feet and sleeping overnight above 11,000 feet.