The Tunneltal, around 7 kilometres long and 0.2 to 2 kilometres wide, lies on the Rahlstedt/Ahrensburg axis and includes the lowlands through which the Wandse, the Stellmoorer Quellfluss and the Hopfenbach flow.
The Tunneltal is completely protected. Strictly speaking, there are two adjacent nature reserves:
The Hamburg part, the Stellmoorer Tunneltal nature reserve, is 202 hectares in size, while the Stellmoor-Ahrensburger Tunneltal nature reserve in Schleswig-Holstein has an area of 339 hectares. The Tunneltal forms a natural unit with the adjacent Höltigbaum nature reserve. Due to its archaeological and geological uniqueness, this area is particularly worthy of protection.
The Tunneltal was formed towards the end of the last Ice Age (around 15,000 years ago) by flowing meltwater that made its way through a tunnel under the ice. The landscape created by this is now characterized by hedgerows, wet meadows and wooded areas. With its dry grasslands, coppice forests and wet biotopes, the Tunnel Valley is an important refuge for plants and animals threatened with extinction.
The Tunnel Valley is known for its unique concentration of prehistoric sites from the late Paleolithic period. Excavations by Alfred Rust in the 1930s have revealed stone, wood and bone tools used by reindeer hunters as well as large quantities of hunting spoils.
In the northwestern part of the Hagen forest you can find the remains of the medieval Arnesvelde Castle, which existed until 1594. The walled main castle with its oval floor plan is located in the Hopfenbach valley. On a tongue of land that projects into the valley from the east is a spacious outer castle protected by a section wall and moat.