Mount Everest is Earth's tallest mountain — towering 5.5 miles (8.85 kilometers) above sea level — and is actually still growing.

While it and the rest of the Himalayas are continuing an inexorable uplift that dates back to their birth roughly 50 million years ago, when the Indian subcontinent collided with Eurasia, Everest is growing more than expected from this event alone.

Scientists now think they know why, and it has to do with the monumental merger of two nearby river systems.