There is a notion that radiation may be limiting qubit lifetime. Even if you use low-radiation materials and shielding, you still have to worry about muons generated by cosmic rays. They are very penetrating – some reach kilometers down into the rock.
If so, we may need to build quantum computers several klicks underground. The deeper, the better: another factor of ten attenuation of the muon flux could materially extend qubit lifetime and drastically improve performance.
We could perhaps use the old Homestake mine in South Dakota, now the location for DUSEL (Deep Underground Science and Engineering Laboratory ). But it’s only 2.4 km down.
The best mines ( for our purpose) are more than 3 kilometers deep, and a few approach 4 km. The problem is, of all the ten deepest mines in the world, only one is in the US.
The United States faces a mine shaft gap.