How Much Detail from a Severely Underexposed D800 Photo?

More Nikon D800 fun. Just to see how far I could take a Nikon D800 image, I processed this black and white from a single dramatically underexposed shot. The processed photo is above and the unprocessed, underexposed shot below.
Nikon D800, 14-24mm f2.8. Processed in Adobe Camera Raw, then Nik Color Efex Pro 4 using Detail [...]