[orbital-decay]

I don't think you need a mountain for that. A properly guarded surface complex with enclosed corridors between facilities would be far cheaper and enough to protect something from intruders or satellite surveillance. IIRC similar facilities (Cheyenne Complex, Zheleznogorsk MCC, Yamantaw) were built as nuclear strike resistant bunkers first. There are also nuclear waste storage facilities all over the world.

[gemstones]

But a mountain would work, yes? And if you already need the mountain lab for physics, what is the extra cost to staff military personnel there?

[jmnicolas]

Digging holes 2400 meters deep isn't exactly cheap, even for a country like China.

[LargoLasskhyfv]

They didn't dig 2400m 'deep'. They dug into the sides of mountains which rise 2400m upwards from where they dug, as a side a effect from a construction site for water tunnels and their access-ways which would have been dug anyways for the adjacent dam and electricity generation site.

AFAIK nobody goes 'that deep' for science alone, it's always adjacent to mining, leftofer/unused/abandoned tunnels, and often closes down when the mining ceases, and the operation getting too expensive without the ongoing mining.

See https://en.wikipedia.org/wiki/SNOLAB , https://en.wikipedia.org/wiki/Canfranc_Underground_Laborator... , https://en.wikipedia.org/wiki/Sanford_Underground_Research_F... for some examples.