Which is the deepest river gorge on Earth by some measures?

The Yarlung Tsangpo Grand Canyon in Tibet, where the river runs more than 5,000 metres below the peaks on either side - roughly three times the depth of the Grand Canyon, and by most accounting the deepest gorge anywhere.

It exists because a river is older than the mountains it cuts through. The Yarlung Tsangpo runs east along the north side of the Himalaya for 1,700 km, then turns hard around the massif of Namcha Barwa and drops through the gorge into India, where it becomes the Brahmaputra. The mountain rose fast; the river cut down faster, which is why the canyon is there rather than a diverted river somewhere else.

The title is contested, because depth depends on where you decide the rim is - measure from the highest peak within some arbitrary distance and you get one ranking, measure the width-to-depth profile and you get another, and Peru's Cotahuasi and Colca canyons and Nepal's Kali Gandaki are all argued for. The inner gorge was not fully traversed until 1998, and an unclimbed 30-metre waterfall inside it was only confirmed by a Western expedition in 1998 after decades of rumour.