If it's like a big empty hole at a secretive mil site, then one likely possibility is an EMP test site, to test EMP resistance.
In old mil themed books, I have seen a B-52 sitting atop wooden scaffolding with dug-out surroundings for such testing. It's an outdoor makeshift isolation scheme for testing oversized items. Indoors, an anechoic chamber is used for EM testing.
If it's like a big empty hole at a secretive mil site, then one likely possibility is an EMP test site, to test EMP resistance.
In old mil themed books, I have seen a B-52 sitting atop wooden scaffolding with dug-out surroundings for such testing. It's an outdoor makeshift isolation scheme for testing oversized items. Indoors, an anechoic chamber is used for EM testing.