If internet is down, obviously Bitcoin will be in chaos,  but am sure developer will surely go for alternative, like intranet but i wonder if it is possible, so more probably they will seek a transmissible frequency to re-establish bitcoin network.  Or maybe thru infrared, bluetooth.  Somehow I read an article about someone developing a way to trarnsmit bitcoin transaction thru bluetooth. So maybe it is possible to re-establish network using these methods.