The most pressure it would experience would be the difference in internal vs external pressure. At 1000ft of depth there’s a pressure of 440psi. Assuming the sphere somehow managed a perfect vacuum that’s still well below the 6000psi compressive strength of high strength concrete, hell they would still have more flexural strength. The spheres themselves definitely wouldn’t be the weak link.
The most pressure it would experience would be the difference in internal vs external pressure. At 1000ft of depth there’s a pressure of 440psi. Assuming the sphere somehow managed a perfect vacuum that’s still well below the 6000psi compressive strength of high strength concrete, hell they would still have more flexural strength. The spheres themselves definitely wouldn’t be the weak link.