The Millennium Gate was a 21st century structure and Earth's first self-sustaining civic environment.

The Millennium Gate was built in Portage Creek, Indiana, United States of America in the early part of the 21st century. Its construction was finished around the year 2012.

The Millennium Gate was one kilometer high, had a base width of 3.2 kilometers and could be seen from space. The Gate itself was covered in highly-reflective solar panels and was built as a self-contained biosphere with its own ecosystem as well as with over six hundred stores. Eventually, the Millennium Gate would serve as a model for the first colony on Mars.

The building of this immense project was scheduled to start in 2001 after the necessary legalities were finished before midnight on the last day of 2000. The Texas company behind the project appointed Gerald Moss as the Millennium Gate spokesman. The reason for building the Gate in Portage Creek was that its city administrators offered the company free building permits, new roads and deferred local taxes. Local citizens were offered twenty percent above the current market value for their properties. The catch was that all citizens needed to agree to sell; if even one of them refused, the whole project would fall through and terminate, and then the Millennium Gate would be built on its alternate location in Canton, Ohio.

In general, the people of Portage Creek welcomed the building of the Millennium Gate. However, there were those who vehemently opposed the structure, believing that claims about it being a self-sustaining civic environment were mere propaganda and that it would be nothing but a glorified shopping mall. One such individual was Henry Janeway. He owned a little bookstore, Alexandria Books, in Portage Creek and was the last hold out, preventing the project from becoming a reality. The deadline for the citizens of Portage Creek to agree to surrender their properties was to pass at midnight, December 31, 2000.

Moss and Shannon O'Donnel, an ancestor of Kathryn Janeway's, managed to convince Henry Janeway to leave his bookstore before midnight and on the first day of January 2001, the building of the Millennium Gate commenced.

Captain Kathryn Janeway found out that – unlike what she had believed all her life – her ancestor Shannon O'Donnel was not instrumental in the building of the Millennium Gate but a mere engineering consultant on the project. However, what Captain Janeway did discover was that Shannon O'Donnel did play an instrumental role in Millennium Gate's construction, when she convinced Henry Janeway to leave his shop and no longer protest the building of Millennium Gate, which was about to be scrapped as a result of his stubborn refusal to sell. (VOY: "11:59")