Over the centuries, millions have made the perilous journey from South Asia across the Bay of Bengal to South-east Asia in search of trade, employment and better lives - sometimes voluntarily, sometimes driven or forced by circumstances.
Every year, thousands of the near-destitute, usually stateless minority Rohingya on both sides of the Myanmar-Bangladesh border - and poor Bangladeshis too - board smugglers' boats for a few US dollars or even free of charge, having been told they can pay later.
For most, the destination is Malaysia, which until this week had been accommodating.
But in Thailand, which is supposed to be just a way station, the migrants find themselves sucked into human trafficking networks that hold them captive in boats offshore, or in squalid makeshift camps in the jungle, to extort money from their relatives.
Many migrants disappear without a trace. The Thai authorities have in recent days found nameless corpses in dozens of shallow graves in clearings in the tropical jungles of the south, where the migrants had been held by traffickers.
Thousands more boat people from Myanmar's troubled Rakhine state and Bangladesh's Teknaf area, both dogged by poverty, have turned up in Thailand, Malaysia and Indonesia. Activists who track the annual migration say there are thousands more on boats offshore.
The United Nations refugee agency, UNHCR, has said that an estimated 25,000 Rohingya and Bangladeshis boarded boats to cross the Bay of Bengal between January and March - almost double the number in the same period last year.
"People are unaware that money will be extorted from them later in the journey, and what started with being smuggled soon turns into trafficking in people," the UNHCR said.
Dr Surin Pitsuwan, a former Asean secretary-general, said in a telephone interview that he had long warned that the exodus could "put pressure on the cultural and social fabric of Asean".
Asean and the multilateral "Bali Process" on People Smuggling, Trafficking in Persons and Related Transnational Crime have tried to produce an orderly system to deal with undocumented migration, but it has not worked.
Now, Thailand has called a regional meeting on May 29 of senior officials from 15 countries, including Myanmar, Malaysia, Indonesia and Bangladesh, to "work together to address the unprecedented increase of irregular migration".
Thailand has few options. It faces being left in the lowest tier of the US State Department's influential annual "Trafficking in Persons" report due out next month, and a potential European Union ban on Thai seafood over slavery in the fishing industry.
The discovery of a human trafficking network that involves state officials and has, for years, preyed on migrants does Thailand's credibility little good.
"Countries of origin, transit and destination must work together... by addressing the root causes," its Foreign Ministry said in a statement.
The issues have been complicated in modern times by nationalism and domestic politics.
Naypyitaw, for example, is unlikely to bring any breakthroughs to the meeting as it does not recognise the Rohingya as an official ethnic minority.
Bangladesh has been bitter about this. "Their policy is Burma for the Burmans. As simple as that," a senior Bangladeshi government official told The Straits Times on the phone from Dhaka, on condition of anonymity, using Myanmar's former name.
The Myanmar government and local Buddhist Rakhine view the Rohingya as illegal "Bengali" immigrants. They have been subjected to discrimination and violence that has driven over 100,000 into camps for the internally displaced. Previous waves have fled to Bangladesh, where well over 200,000 live in refugee camps.
But the real roots of the problem lie in colonialism. Migration between South Asia and South- east Asia has always been common; it was described by Dr Sunil Amrith in his 2013 book, Crossing The Bay Of Bengal, as "one of the largest movements of people in modern history".
"Until the 1930s, you had virtually nothing like migration control," Dr Amrith said in an interview from London, where he teaches modern Asian history at Birkbeck College.
"Then immigration controls became universal across Asia, and they were overlaid quite awkwardly on patterns of migration that far pre-dated the 20th century."
In colonial times, Bengal was partitioned first in 1905 and then in 1947. Meanwhile, Burma was ruled as a province of India by the British, but they agreed to rule it separately in 1937 - in effect, another partition. When colonialism ended and new borders were set, many people who had moved fluidly across regions found themselves accidental foreigners.
The Rohingya are the residue of this history. The region needs to develop ways to manage the undocumented migrants in a humane and constructive manner - and crack down on the criminal gangs that prey on them.
The senior officials meeting in Bangkok, and their ministers in their respective capitals, will have to rise to the challenge or risk even worse problems ahead.
