Need something more original than the standard underground base.

[Ursa major]

The best place to hide is under your enemy's noses.

It is said that this sort of thing is how Tutankhamun tomb remained hidden for so long (it probably being intended for someone else (someone a lot less important) and built/excavated underneath another tomb).

Also:

Spoiler: Seminal spy film whose name I won't put in the spoiler title

The STENCH HQ was hidden under the offices of the Secret Service in Carry on Spying. To be fair, it was a rather long facility: the far end of it was under a field, if I recall correctly (which is far from certain).