Kra Isthmus

The Kra Isthmus marks the boundary between two sections of the mountain chain which runs from Tibet through the Malay peninsula. The southern part is the Phuket Range, which is a continuation of the Tenasserim Hills, extending further northwards for over 400 km (250 mi) beyond the Three Pagodas Pass.

The Kra Isthmus is in the Tenasserim–South Thailand semi-evergreen rain forests ecoregion. Dipterocarps are the dominant trees in the ecoregion.

Ancient trans-peninsular route

The isthmus carried traffic between the Bay of Bengal and the Gulf of Thailand long before the canal proposals. Bellina and her co-authors give the crossing at Chumphon and Ranong as about 50 km wide. They report Wheatley's estimate that taking it saved about four months on a journey between South Asia and China, twelve months against sixteen. Journey times over the isthmus varied from a week to a month with the season and the route, and combined porters, wagons, pack animals, bull carts, boats, rafts and pirogues. The same authors hold that the routes were in use before the Mauryan and Han unifications, for exchange among independent polities around the two basins. On their account this was before the peninsula began to be circumnavigated, which they date to the 1st century BCE at the earliest. They present this early use as a conjecture, resting on the Qian Han Shu notice of a mission sent by Emperor Wu (r. 140–87 BCE) to Huangzhi.

Chinese records of the 3rd to 7th centuries describe a polity, Tun Sun, astride the route. The Liang shu calls it a mart where east and west met, with more than 10,000 people a day, and, in Wheatley's translation, a place lying on an ocean stepping-stone, where one crossed from one sea to another. Where Tun Sun lay is disputed, and the isthmus is one of the positions proposed for it. Pelliot read the Chinese notices as evidence that goods were carried across the isthmus, because Chinese junks did not yet sail directly from the coast of Annam to the Straits. Wheatley himself held that Tun Sun occupied the whole breadth of the isthmus, since it was said to communicate with countries beyond both the Bay of Bengal and the South China Sea. Kenneth R. Hall likewise treated Tun Sun as the terminus of the portage. He listed Langkasuka, Tambralinga and Takkola beside it as the major entrepôts of the age, settlements that grew up because of the crossing and depended on it. He described them as river-valley polities financed almost entirely by trade revenue, against which Funan, with ricelands as well as ports, had the broader base. Miriam Stark counts more of them, with a condition attached. Perhaps ten or more coastal centres may have developed along the peninsula's coasts in this period, if the Chinese reports are taken at face value.

The crossing lost its importance in the fourth century AD. Sing C. Chew relates the change to the Chin dynasty's loss of the overland routes west, which turned Chinese demand towards the sea. Ships willing to spend about six weeks working through the Straits of Malacca could then sail on across the South China Sea to the southern Chinese ports. That route took them past both the portage and the ports of Funan on the coasts of Cambodia and Vietnam.