A stunnel was a portmanteau of Stellar Tunnel, an interstitial connection which could be used for interplanetary or interstellar transport.
The Interstitial Mass Transit System planned to create a stunnel to connect Mars with Arcturus. (PROSE: Transit)
When the Daleks attacked Pluto's moon, Charon, a group of human scientists researching the abandoned technology managed to create a stunnel to escape. (PROSE: GodEngine)