"Crossing the Bay of Bengal," by Sunil S. Amrith Piyal Adhikary/European Pressphoto Agency A Hindu festival in Gangasagar, India, 2008. A few miles up the road from my home, on a sandy beach in the Indian state of Tamil Nadu, there is a crumbling fort that extends into the Bay of Bengal. The fort was built in the 17th century; over the years, it has been controlled by Mughal rulers from the north, by the French and by the British. Farther down the coast, past a string of fishing villages, sits the former French colony of Pondicherry, its wide boulevards and elegant villas overlooking the shimmering waters of the bay. A few miles away is an ancient Roman trading outpost. And two and a half hours beyond sits the former Danish port town of Tranquebar, its yellow fort blending into the sand, its rusty cannons pointing at the ocean. The diversity of this region - a triangle of flat land that extends along the southeastern coast of India - is testament to an ancient history of migration, imperial conquest and seaborne commerce. As Sunil S. Amrith writes in his fascinating new book, "Crossing the Bay of Bengal," the countries bordering the bay have for centuries been home to a cosmopolitan world that "is strangely familiar from the vantage point of the early 21st century - a world of polyglot traders and cross-cultural marriages, a world in which long-distance travel is a common experience." Lacking a political union to give it coherence, this world has often been overlooked. Although several books have been written about the strategic and geopolitical significance of the Indian Ocean - Robert D. Kaplan's "Monsoon," for instance - there is little awareness of the cultural and historical ties that bind diverse nations bordering the bay. Amrith's signal achievement is to bring these ties to light. In doing so, he gives voice - and an identity - to one of the most complex and culturally interesting regions of the world. Geographically, the Bay of Bengal stretches from Sri Lanka up the coast of eastern India; it curves under Bangladesh and Myanmar and then heads south along Thailand and Malaysia until it reaches the northern coast of Sumatra, in Indonesia. It is the largest bay in the world; one-quarter of the world's population lives in the countries fringing it. The bonds that hold together its various nations and people were forged in the crucible of migration. Amrith, a professor of history at the University of London, writes about wave after wave of warriors, explorers and laborers who crossed the bay. We meet Arab traders and Chinese merchants; Portuguese, Dutch, French and British colonizers; and, of course, the masses of workers who spilled out of South India and Sri Lanka to build the rubber, coffee and spice plantations of Southeast Asia. Statistics are unreliable, but Amrith estimates that roughly the same number of people traveled the bay during the late 19th and early 20th centuries (the period of peak migration) as made the trans-Atlantic crossing. Not all these journeys were voluntary. "Mobility embraced captivity," Amrith writes, and he vividly describes the harsh conditions endured first by prisoners banished to distant colonial jails in places like the Andaman Islands and then by the indentured laborers who worked on the plantations, their exertions connecting the bay to the global economy. In the early 20th century, Malaya, whose rubber fueled an auto boom halfway around the world, was among the most valuable possessions in the British Empire. Some decades later, when the Great Depression hit, migration across the bay dipped too. The result of these successive waves of migration is what Amrith (borrowing from the American historian Daniel K. Richter) calls "layered pasts": rich multicultural histories that made cities in the region some of the most diverse in the world. Traces of these histories linger today in a shared culinary repertory that will find you eating roti in India and Malaysia; in the synagogue, churches, mosques and monasteries that jostle for space in Yangon; or in the colorful South Indian temples of Singapore. These cities were precursors to multiethnic, 21st-century metropolises like New York and London. They were home (and, in many cases, continue to be home) to a cosmopolitanism that I, after reading the book, have come to think of as a distinctive culture of the bay. Akash Kapur is the author of "India Becoming: A Portrait of Life in Modern India."