The world’s largest coastal mangrove forest is the Sundarbans, which stretches for almost 15,540 km² (6,000 miles²) across India and Bangladesh, and acts as a natural barrier against tsunamis and cyclones that often blow in from the Bay of Bengal. With saltwater-tolerant roots, this forest’s mangrove trees sometimes exceed 21 m (70 ft) in height above islands of layered sand and grey clay, which have been deposited by rivers that flow for over 1,000 miles from the Himalayas to the Bay of Bengal.