From the Depths
Strongholds are safe havens usually built around massive radio towers. A small group of Russians were the first to discover that the husks could be ‘shut down’ by radio signals. The power grids had long been down at this point so they converted an old radio tower to run off of solar energy, eventually boosting the signal high enough to set up about a ten mile perimeter.
There is no way to be sure, but it’s believed that other strongholds exist in Canada, Chile, and South Africa.