The mainland comprises three distinct geographical regions and an extraordinary variety of habitats and ecosystems. In the sierra , mountain paramos, snow-tipped volcanoes and the Andean mountains form the north-south spine of the country. To the east, they slope down through primeval cloudforests into the tropical rainforests of the Oriente and the Amazon basin, forming an unassailable wall that prevents the moist air from heading west, ensuring the rainforests have high levels of precipitation throughout the year. On the west of the Andes, the coast ( costa , or litoral ) comprises dry and tropical forests, lowland hills and a shoreline of expansive beaches and mangrove swamps.