Fishermen row their boats near a mangrove plantation area at Mathurakhand village in Sunderbans, about 125km (77 miles) southeast of the eastern Indian city of Kolkata. The Sunderbans, a 26,000sq km (10,000 square mile) area of low-lying swamps on India's border with Bangladesh, is dotted with hundreds of small islands threatened by rising sea levels. REUTERS

Sea levels are rising more than twice as fast as the global average in the Sunderbans, a low-lying delta region comprising 200 small islands in the Bay of Bengal and inhabited by around 13 million impoverished Indians and Bangladeshis.

Scientists predict much of the region could be underwater in 15 to 25 years, forcing the largest ever human migration in history.

With a total 13 million inhabitants, the Sunderbans region is more populated than any island nation, reports AP.

"This big-time climate migration is looming on the horizon,'' said Tapas Paul, a New Delhi-based environmental specialist with the World Bank, which is assessing and preparing a plan for the Sunderbans region.

Already four islands have gone underwater and many others abandoned due to sea rise and erosion.

"The chance of a mass migration, to my mind, is actually pretty high. India is not recognizing it for whatever reason,'' said Anurag Danda, who leads the World Wildlife Fund's climate change adaptation programme in the Sunderbans.

"It's a crisis waiting to happen. We are just one event away from seeing large-scale displacement and turning a large number of people into destitutes.''

Thousands have already been left homeless.

Each year, the inhabitants build mud embankments to protect their crops from saltwater and animals. But each year the flooded rivers and monsoon wash away their efforts.

Dams and irrigation systems add to the problem as they block the sediments that build up river deltas.

Saltwater long ago claimed the five acres where Bokul Mondol once grew rice and tended fish ponds. His tiny hut sits at the edge of the water now.

This is the fifth mud hut he has had to build in the last five years as the sea creeps in.

A 2013 study by the Zoological Society of London found the Sundarbans coastline retreating at about 200 metres (650 feet) a year.

The Geological Survey of India says at least 210sq km (81 square miles) of coastline on the Indian side has been lost in the last few decades.

With no national plan to relocate residents of the Sunderbans, the West Bengal administration is seeking international help in addressing the imminent problem of relocation and protection of the region.

Bangladesh too is yet to do anything about the problem, besides building some dykes and barriers on some of the islands.

The Sundarbans (meaning beautiful forests) region is home to rare wildlife, including the world's only population of mangrove forest tigers.

A Unesco World Heritage Site, the Sunderbans spans over 20,000sq km in total, across India and Bangladesh. The Indian Sunderbans forms the largest tiger reserve and national park in India and is home to rare bird species like the Masked Finfoot, Mangrove Pitta and the Mangrove Whistler.

Part of the world's largest delta formed by the rivers Ganges, Brahmaputra and Meghna, Sunderbans is also the world's largest estuarine forest.

The mangrove forests act as a natural buffer protecting India's West Bengal state and Bangladesh from cyclones.

Most scientists believe that the long-term solution is to encourage people to leave, so that the region naturally rejuvenates and the mangroves help capture sediments over time.

"We have 15 years ... that's the rough time frame I give for sea level rise to become very difficult and population pressure to become almost unmanageable,'' said Jayanta Bandopadhyay, an engineer and science professor at Jawaharlal Nehru University in New Delhi.

However, most of the residents who have been living on the land since the early 1800s are reluctant to leave. This is despite the rising water levels and tiger numbers. The region has around 200 tigers and tiger attacks have been on the rise.