The Himalayan Wall

For a long time, the Indian monsoon was explained by the Tibetan Plateau: in summer the high plateau was thought to heat up fast and act as a giant engine, pulling moist air off the Indian Ocean. That picture has been challenged. A landmark 2010 study (Boos & Kuang) found that flattening the plateau barely weakens the monsoon, as long as the mountains to its south stay in place.

The bigger player turns out to be the Himalayas themselves. They work less like a pump and more like a wall of insulation: they block cold, dry air from the north out of the picture, so hot, humid air can pile up over India through the summer. That trapped warm, moist air is what powers the monsoon's huge inflow of rain over the subcontinent.