I think the the popular theory that they used it to make the tunnels resistant to heat, water, and electricity/radiation from the last time the sun was in a solar minimum and the electromagnetic field of earth was weak. giving the humans (?) in the tunnel protection, is a good one.

I think the the popular theory that they used it to make the tunnels resistant to heat, water, and electricity from the last time the sun was in a solar minimum and the electromagnetic field of earth was weak. giving the humans (?) in the tunnel protection, is a good one.