The Millennium Gate was a kilometre-high self-sustaining tower built in Portage Creek, Indiana, in the early twenty-first century, which Kathryn Janeway believed her ancestor Shannon O'Donnel had helped to create.
The Gate is the subject of the fifth-season episode 11:59. Covered in reflective solar panels and containing its own enclosed ecosystem, it could be seen from orbit, and it later served as the model for the first habitats on Mars.
Janeway grew up on stories of O'Donnel as an astronaut and visionary engineer behind the project. Flashbacks to the last days of 2000 show a different picture: an out-of-work consultant whose car breaks down in Portage Creek, where bookshop owner Henry Janeway is the last holdout refusing to sell his premises to the developers.
She persuades him to give way just before the deadline of midnight on New Year's Eve, and stays. The tower was completed in 2012.