These table-top mountains are the oldest exposed rock formations on the planet, the remains of a large, sandstone plateau that once covered the granite basement complex between the north border of the Amazon Basin and the Orinoco, between the Atlantic coast and the Rio Negro. Throughout the course of the history of Earth, the plateau was eroded, and the tepuis were formed from the remaining monadnocks. There are 115 such mesas in the Gran Sabana in the south-east of Venezuela on the border with Guyana and Brazil, where the highest concentration of tepuis is found. The precipitous mountains tower over the surrounding area by up to 1,000 meters. On the top of the mountains grow various types of forests with a wide variety of orchids and Bromeliads species. Because of their great age, some tepuis exhibit surface features and subsurface caves that are typical of more water-soluble rock such as limestone. Caves here include the 671m deep Abismo Guy Collet, the deepest quartzite cave in the world. Some of the mesas are pocked with giant sinkholes up to 300 metres in diameter and with sheer walls up to 300 metres deep. These sinkholes are formed when the roofs of tunnels carved by underground rivers collapse.
The plateau of the mesas is completely isolated from the ground forest, making them ecological islands. The altitude causes them to have a different climate from the ground forest. The top presents cool temperatures with frequent rainfall, while the bases of the mountains have a tropical, warm and humid climate. The isolation has led to the presence of endemic flora and fauna through evolution over millennia of a different world of animal and plants, cut off from the rest of the world by the imposing rock walls. Some tepui sinkholes contain species that have evolved in these "islands within islands" that are unique to that sinkhole. The tepuis are often referred to as the Galápagos Islands of the mainland, having a large number of unique plants and animals not found anywhere else in the world. The floors of the mesas are poor in nutrients, which has led to a rich variety of carnivorous plants. The weathered, craggy nature of the rocky ground means no layers of humus are formed.
The tepuis, also known as 'islands above the rainforest', are a challenge for researchers, as they are home to a high number of new species which have yet to be described. A few of these mountains are cloaked by thick clouds almost the whole year round. Their surfaces could previously only be photographed by helicopter radar equipment. Humans have still yet to set foot on many of the tepuis.
A few of the most notable of the 115 Tepuis: