Lots of "soft disclosure" in that movie, including observation from the "moon office" showing there is something weird about the moon, restricted travel to certain parts of the planet, and the shape of the "studio" that Truman is inside (snowglobe type of structure with level land and water) with an outer dome "firmament" type structure.

Note that the Encyclopedia Americana from 1949 or late 50s states that at one point near Antarctica (icewall) the firmament is at a height of 13,000 ft, if I recall correctly. It appears to be some sort of electromagnetic field.

Lots of "soft disclosure" in that movie, including observation from the "moon office" showing there is something weird about the moon, restricted travel to certain parts of the planet, and the shape of the "studio" that Truman is inside (snowglobe type of structure with level land and water) with an outer dome "firmament" type structure.