The story was partially filmed at Wookey Hole Caves, a location renowned for bad luck. As if to confirm local superstition about the site, an electrician broke his leg, Elisabeth Sladen's motorboat went out of control, and one of the stuntmen was injured getting her out of the water and ended up in hospital.
This episode was watched by 8.3 million viewers on its original transmission.
The crew of Nerva use Ingram Mac-10 machine pistols as their weapon of choice.