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The Underwater Musing. Aka, All About The Base. (Sorry. I'm so sorry!)
Similarly, this is not first time the possibility of ghosts has been raised in Doctor Who. In the 1972 story, Day Of The Daleks, the Third Doctor was sent to Auderly House, to investigate ghostly sightings which later transpired to be time-travelling soldiers from the 22nd century. In recent years The Ninth Doctor faced Gelth in The Unquiet Dead, while the Tenth Doctor story Army Of Ghosts featured ghostly apparitions, which were subsequently revealed to be a Cyberman invasion force.
Whether intentional or otherwise, eyes appear to be a recurring theme of some importance in series nine. The Magician’s Apprentice and The Witch’s Familiar both made strong use of eye imagery, from the hand-mines, to the revelation that Davros has semi-functional eyes. Under The Lake has several close-ups of the mysterious symbols, etched into the wall of the spaceship, reflected in the eyes of various characters, including the Doctor; magnets, which imprint a memory into the synapses of the brain. The Doctor refers to these as the equivalent of an “earworm”, and are, courtesy of Peter Andre, one of the reasons he no longer has a functioning radio aboard the Tardis which he dismantled to build a clockwork squirrel!
The cliff-hanger suggests a degree of complexity to the next episode, with the climax featuring the Doctor leaving The Drum and travelling back in time to investigate the origins of the craft. Could it be that the stasis pod contains The Doctor himself?