The DSV 4600 Middlesub is a submersible which was available in the 1960s, and has seldom been used since. The Middlemen (present and Guy Goddard) and Wendy Watson used the sub to reach The Candle's hideout on Snake Island.
The Middlesub has a protective layer of asbestos, which is the reason for its discontinued use. Wendy refers to it as "the cancersub".