When you posted "40,000ft" you were on an aeroplane, so I think we all assumed it was an aerial clue.
Is the 40,000ft related to depth? Perhaps the missing submarine?
Depends on the depth of the tunnels. 40,000ft is 7.5miles. I am not sure we have anything that deep (aside from mines, etc.) on Earth.
Ah. Okay. I have only recently gotten involved, so I am still trying to catch up with all the posts.