[en] Due to its extremely low area dose rate, the Underground Laboratory for Dosimetry and Spectrometry (UDO) of the PTB at the 925 m level of the Asse II Salt Mine offers unique possibilities for the investigation and calibration of dosimetry systems of high sensitivity as are used, for example, in environmental monitoring. Due to its low area dose rate, this laboratory has an outstanding position worldwide. The low ambient dose equivalent rate in the UDO of approx. 1 nSv/h, that means of only approx. 1 percent of the ambient dose rate typically encountered at the Earth's surface, is mainly due to the following reasons: - At the depth at which the UDO is situated, the penetrating muon component of cosmic radiation which considerably contributes to the environmental equivalent dose rate at the Earth's surface (in Braunschweig, for example, approx. one third) is already attenuated by more than five orders of magnitude and is therefore completely negligible for dosimetric investigations; - The activity concentration of the pure rock salt surrounding the UDO is extremely low; it amounts only to a few becquerel per kg (from ⁴⁰K), which is approx. one hundredth of the values usually found for 'common construction materials'. Uranium and thorium have not been detected so far (upper limits for U and Th: 0,1 Bq/kg and 0,01 Bq/kg, respectively); - The radon concentration of the air is about 10 to 20 becquerel per m³ and stems from the ventilation of the mine with outside air; - The laboratory building consists of selected materials with very low natural activity. The PTB is thus the only National Metrology Institute capable of investigating dosimetry systems free from the disturbing influences of natural ambient radiation. The objective of the present report is to illustrate, by means of selected examples, the activities which have so far been carried out at the UDO and which are planned for the few remaining years to come. The final chapter is devoted to whether the PTB will really need an underground laboratory in the future and what the prospects for it are like. (orig.)