The most obvious message on the wall was written at almost exactly head height. Someone-presumably the Doctor- had found a bright red crayon and written in huge, shaky letters-
ACE-
THIS DOOR MUST NOT BE OPENED USING NITRO-9. OR A BASEBALL BAT. UNDERSTOOD?
This was followed by someone else's handwriting-
Aaw, Professor... I haven't even made any recently!
Then what were you doing in the laboratory yesterday?
um... Cooking.
Good girl. Blow up that door instead.
Clara decided that it was best not to ask where 'that door' was.