- Prop Recycling: The Flying Submarine and the Seaview from Voyage to the Bottom of the Sea are reused (the latter in the background of the some of the wide shots), as well as several set props from The Time Tunnel. More tragically, both the Gemini II and Jupiter II models from Lost in Space had windows chopped in their hulls and used to top a couple of skyscrapers.
- Special Effects Failure: During the climatic fight, Matthews throws the villain against one of the stacks of gold. It visibly wobbles, indicating it's just a wood frame painted gold.
Trivia / City Beneath the Sea