Tomorrowland is a special dimension in time and space that resembles a futuristic version of Earth.
The dimension that came to be known as Tomorrowland was first discovered in 1899 by Nikola Tesla and other Plus Ultra scientists, who had been seeking a private locale to test and experiment new ideas and technologies with minimal interference. An effort to open a portal into this new dimension began in earnest and in the Siberian wilderness in 1908, a super-powered nuclear bomb was used to open a hole in space and time to access the dimension, causing the Tunguska Event. It worked, but Plus Ultra sought less destructive means of travel between the two realms. In 1928, a particle beam created by Nikola Tesla allowed for less damaging means of travel, but an experiment gone awry the following year lead to a delay in human settlement. However, construction by robotic workers continued within the other dimension to create powerstations and infrastructure for Plus Ultra's needs and by the mid-30s, Plus Ultra was ready to resume their work.
They planned a reveal of the alternate dimension at the 1939 New York World's Fair and a contest to name the realm. The city was to be held as a publicity stunt, though Howard Hughes, one of Plus Ultra's biggest backers, had already been insistent on referring to the alternate dimension as "The New Frontier". However, a conflict between Plus Ultra, a group of Nazis lead by scheming ex-Plus Ultra scientist Werner Rotwang, and Henry Stevens, a young Plus Ultra inductee turned vengeful cyborg, lead to the reveal plan being shelved and numerous setbacks delaying any further work.
The beginnings of the city known as Tomorrowland itself would result from the work of a redeemed Henry, who built new robot workers and lead them to begin constructing the city before his power sources drained for good in the mid 1950s.