The largest freshwater lake in Southeast Asia, Tonle Sap is of great ecological and economic import, designated as a UNESCO biosphere in the late nineties and the main source of fish for Cambodia.

This fascinating lake changes in size dramatically twice a year, with it's total surface area expanding from 3000 square kilometres to 16000 square kilometres at the start of monsoon season, then shrinking back again a few months later. Monsoon season causes the Mekong river to flood the lake with extra water, making it multiply in size and depth. At the end of the season the tide turns and the water starts to flow back out of the lake and along the Mekong, leaving a smaller, shallower lake behind.

It is also a fascinating cultural destination for visitors who will discover the great floating village of Chong Kneas. As the lake swells in size, the village along the edge of the lake is moved and rebuilt further out, along the new shoreline, while other residents cope with this phenomenon by building floating homes which need remooring. Many of the inhabitants of Chong Kneas rely on the rich and varied marine life of the lake to fish and sell, as well as to feed their families. A large service industry has also built up, where boat stalls, selling fresh produce as well as services, navigate the 'streets' of brightly coloured homes which make up the village, plying their trade.

The lake itself is, most of the year, a large, shallow and placid place, dotted with fishing boats. There are speedboats and large 'bus' boats for hire and it is worth travelling towards the centre of the lake, away from the busy shoreline, to find a peaceful body of water, bordered by rich green vegetation, fertile farmland and a vast blue horizon.