MENALAUS's image is of the reactor service compartment aka the 'Tunnel' of HMS Sceptre, and I would take the late VAdm Coward at his word (as long as it wasn't about aircraft recognition
). So not a tunnel as might be imagined - assuming the Valiant's RSC was similar in layout to the Swiftsures'. Only ever invited onboard an O-boat so no personal experience of SSNs.