Traditional Indian geography looks like the product of stay-at-home minds. In texts from perhaps as early as the fourth century AD, continents radiate from a mountainous core. Around concentric rings of rock flow seas designated ‘sugar’, ‘salt’, ‘wine’, ‘ghee’, ‘curds’, ‘milk’ and ‘water’. Yet the schematic picture and fanciful names conceal real experience of an ocean leading to commercial opportunities – the Sea of Milk took seafarers to Persia, for example, and the Sea of Ghee led travellers to Ethiopia. Older tales of Buddhahood suggest the antiquity of long-range navigation. Pilotage by means of the stars appears in them as a divine gift. The Bodhisattva often features, incarnated as a merchant. He intervenes to save sailors from cannibal goblin seductresses in Sri Lanka. He invents an unsinkable vessel for a pious explorer. In one story, a philanthropist called Sankha, impoverished by his own generosity, resolves to ‘take ship for the gold country, whence I shall bring great wealth’.

The Indian Ocean was the world’s first arena of long-range exchange, traversed millennia before anyone managed to open viable routes to and fro across the Atlantic or the Pacific. The monsoons made this precocity possible: the seasonal winds assured sailors of the prospect of returning home from an outward voyage. It is a rarely appreciated paradox that in the fixed-wind systems of other oceans, ancient explorers liked to sail into the wind so as not to lose touch with home. Naturally, they rarely got very far. Monsoons keep you waiting for the turn of the wind, but once seaborne you can make fast headway.

In consequence, as Barry Cunliffe’s new book says, cultural exchange across Eurasia has been ‘driven by the monsoons’ – perhaps even more than along ‘silk roads’ via deserts or steppes. He focuses on what he calls the ‘underbelly’ of Asia, where India and the Malay peninsula dangle like udders. Three