[capableweb]

How long is a rope?

There is already a bunch of "completely detached" networks out there, organized via wifi links. Freifunk, Guifi and NYC Mesh are three examples of such networks, where you can basically avoid the current internet infrastructure as long as you get hooked up to the mesh network. Lots of interesting services deployed on these networks too :)

[none_to_remain]

I've read about the NYC mesh in the past, I don't remember it being disconnected from the wider internet?

[capableweb]

Usually the meshes have parts of the network that is connected to the internet, and you either automatically get access to those nodes so you can route via them, or you can request access to be able to reach the wider network.

Really depends on the mesh, I don't know the specific answer for NYC mesh.