[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.