In many dielectrics, energetic heavy ions produce thin cylindrical damage zones along their trajectories. Massive physical and chemical changes can occur in these ion tracks with diameters of several nm. In nature, such trails are generated by spontaneous fission of 238U-nuclei. So far, irradiation experiments with heavy ions were always performed under vacuum conditions. Studies of ion-track formation in pressurized solids are expected to contribute to an improved understanding of the creation conditions for fission tracks in the Earth�s crust. Such experiments will be important for dating of geological samples using the fission-track technique. In addition, it is a question of great interest whether the energy deposition of swift heavy ions in a solid, being exposed to extreme pressure, can induce specific phase transitions. This work describes the first experiments on ion track formation under high pressures up to 140 kbar which were performed at GSI by injecting relativistic heavy ions, accelerated with the SIS-heavy-ion synchrotron, through the diamond anvils of a high-pressure cell. It turned out that high pressures can significantly affect the interaction between heavy ions and solids. The effects observed include the suppression of track formation, the complete recrystallization, and the nucleation of new phases
