The Millennium Dome was a well-known landmark in London.
When Ianto Jones joined Torchwood One in 2005, (AUDIO: New Girl) one of his first jobs was at the Millenium Dome, where he saw what was sealed beneath it. The memory made him shudder for years after. (PROSE: Almost Perfect)
In 2050, Inspector Thorne of the Department used the dome as a location for a gateway connected to the Space-Time Manipulator located in Gryffen Manor; he also kept Project Trojan there. K9 Mark 2 confronted him there after he activated his regeneration unit. Alistair Gryffen was forced to use the temporal stabiliser when a strange phenomenon in the STM threatened the Earth. This allowed the vanguard of the Korven Pan-Dimensional Army, led by Lomax, to come through. Gryffen later defeated his agoraphobia, headed to the dome and shouted out the STM's voice-activated termination code, resulting in the STM being destroyed, taking the Korven invaders with it. K9 was attacked by Trojan but managed to defeat him, which also resulted in Thorne's death. (TV: The Eclipse of the Korven)