Since February 2007, DGFI operates a 30m vertical pendulum in the salt mine of Berchtesgaden. The location in Berchtesgaden was chosen because of its very good geological characteristics (good connection of the mountain and the Earth's crust). The not longer aerated part of the salt mine allows to protect the pendulum from air-pressure and temperature fluctuations and the hygroscopic rock keeps the air moisture at a nearly constant low level.
The measured movements of the Earth's crust could have various causes . On the one hand, oscillations of the Earth are excited by external forces and torques and on the other hand by abrupt events like e.g. an earthquake. Elastic oscillations of the Earth are excited by earthquakes with a magnitude bigger than 6.5 on the moment magnitude scale.
In general, the free oscillations are classified according to their moving direction in spheroidal and toroidal modes. Thereby, spheroidal modes describe radial and tangential movements relative to the Earth's crust (only measured with gravimeters and seismographs) whereas toroidal modes describe only tangential movements (only measured with pendulums and strain-meters).