The Sundarbans forest has been in existence for about 4,000 years and has been formed by silt from the Himalayas brought down by the Ganges and Brahmaputra rivers. The Sundarbans waterways rise and fall with the Bay of Bengal tide, with high tide reached every 12 hours and 50 minutes. The Sundarbans of Bangladesh and India covers a total of 10,000 km2, 6,000 km2 of which is on the Bangladeshi side. It is classified as a mangrove forest from the collection of tree types that can survive in this highly saline environment. Mangrove forests support a unique mixture of plants and animals, but unfortunately there is little of this forest type remaining in the world.