Underground facilities small wonder

[wieder]

There is already underground tunnels small wonder that will give one city more defense against all units except diplomatic ones.

Now I wonder if there could be additional SDI-like wonder that would give the city defense against nukes. This would need to be expensive one and as a small wonder it would only apply to one city. The requirement for this "Underground Facilities" would be having the "Underground Tunnels small wonder" in the same city.

This has not yet been implemented. What would be the correct cost and required tech? Maybe mass production?