One of the plot elements that crops up in the movie is a volcanic area that the tunnelers run into about halfway across the Atlantic. This makes sense, as the Mid-Atlantic Ridge is a very active area. The neat part in relation to the movie is that while the existence of a "Ridge" on the seafloor was known at the time, it was not known that it was so active. It wasn't until the Heezen/Ewing/Tharp mapping of the ocean floor in the 1950s that people discovered that Seismic activity and "Seafloor Spreading" due to magma seepage were going on. That was about 20 years after the makers of the film surmised Magma pockets near the Mid-Atlantic.
The film begins in the 1940s and proceeds forward about 20 years.
In the movie's fictional time line, the Channel Tunnel was built in 1940. The real opening date of the "Chunnel" was 1994.
The futuristic-looking car in this film is a 1934 Tatra 77, built in Czechoslovakia.
Walter Huston was in England to make Rhodes (1936), but when that production was delayed, he agreed to play the President in this film to fill in the time.