A pentalium drive as a vital part of a transmat system.
While on the Nerva beacon, Kellman stole the pentalium drive from its transmat so no one could travel to Voga. When Sarah Jane Smith was infected with a virus by a Cybermat, the Fourth Doctor planned to transmat her down to Voga to disperse the virus from her bloodstream. When they got to the transmat terminal, he discovered the drive was missing, but managed to get it working and transmated her and Harry Sullivan to Voga.
Without the drive, the Doctor had no way of getting Sarah and Harry back, so he forced Kellman to tell him where it was. When he had fixed the transmat, he activated it, but nothing came up, so he realized that they had left the area. (TV: Revenge of the Cybermen)