Tonlé Sap, half-lake, half-river, is a genuine geographical marvel, because its flow changes direction according to the season. During the rainy season (May-Oct), the river absorbs the waters of the Mekong as they rise and more-or-less transforms into a lake (its surface is multiplied by 5 or 6 and reaches 16,000km²/9,940mi²). In the dry season (Nov-Jun), the water changes direction and flows back into the Mekong. These constantly shifting conditions are especially problematic for the floating villages, which have to adapt.