The High Fens, which were declared a nature reserve in 1957, are an upland area, a plateau region in Liège Province, in the east of Belgium and adjoining parts of Germany, between the Ardennes and the Eifel highlands. The High Fens are the largest nature reserve or park in Belgium, with an area of 4,501.2 ha ; it lies within the German-Belgian natural park Hohes Venn-Eifel, in the Ardennes. Its highest point, at 694 metres (2,277 ft) above sea level, is the Signal de Botrange near Eupen, and also the highest point in Belgium. A tower 6 metres (20 ft) high was built here that reaches 700 metres (2,297 ft) above sea level. The reserve is a rich ecological endowment of Belgium covered with alpine sphagnum raised bogs both on the plateau and in the valley basin; the bogs, which are over 10,000 years old, with their unique subalpine flora, fauna and microclimate, are key to the conservation work of the park.

Raised bogs, also called ombrotrophic bogs, are acidic, wet habitats that are poor in mineral salts and are home to flora and fauna that can cope with such extreme conditions. Raised bogs, unlike fens, are exclusively fed by precipitation (ombrotrophy) and from mineral salts introduced from the air. They thus represent a special type of bog, hydrologically, ecologically and in terms of their development history, in which the growth of peat mosses over centuries or millennia plays a decisive role. They also differ in character from blanket bogs which are much thinner and occur in wetter, cloudier climatic zones.