I don't believe that one bit. A few things to consider..
An underground nuclear weapon underneath Washington DC? Unlikely.. I think our surveillance will pick that up in a hurry.. It would need to be buried very deep in order for it to not be detected. At which case an underground detonation isn't as effective as a surface detonation. Scary though.. I hope something like this never happens anywhere in the world. We already have enough on our plate.
On another subject.. When the Soviet Union crumbled, there was some nuclear suitcase bombs missing. Have those ever been found?
Whose surveillance would pick it up? I think the point was that it if it was another false flag (or gov't sponsored terrorism) then a nuke could end up practically anywhere. Lots of subway trains run under DC btw.