The Schmalgraf lead-zinc deposit, the deepest Belgian metal mine (290 m deep), was discovered in 1858 at the "Klouster shaft". It is located about 2,500 m southwest of the Altenberg mine (La Calamine).
Exploration began in 1867 after a 500 m long horizontal drainage tunnel (Oskar tunnel) was excavated between the ore deposit (at a depth of 42 m) and the Hohnbach Valley.
The ores were transported from -42 m through the Oskar tunnel directly to the washing plant of the Vieille-Montagne zinc factory of the Altenberg mine. This transport took place on a narrow-gauge railway on the banks of the Hohnbach.
The mine was closed in 1932 due to profound mining problems and a depressed overall economic situation.
The mine had two main shafts, a transport and a pump shaft at a height of -132 m.
In addition, there were three adjacent shafts (260 m west of the first side) for transport, pumping and cables, as well as 2 ventilation shafts. The first of these ventilation shafts went to -132 m and the second to -290 m with built-in fan to -42 m.
Most important was the horizontal exit at -42m (Oskar tunnel) for the ore discharge.
The mine had several levels: -42m, -92m, -132m, -155m, -210m, -255m and -290m.
30 people were employed above ground, 120 miners worked underground.