The South Asian summer monsoon sustains billions of people today. For a long time, the prevailing scientific view has held that the formation and intensification of the South Asian summer monsoon were primarily controlled by the rapid uplift of the Tibetan Plateau. However, geological records present a long-standing puzzle from the Early to Middle Miocene (25 to 15 million years ago): the South Asian monsoon rainfall was remarkably strong, even though the Somali Jet—the primary wind system transporting moisture—was relatively weak.