HMW-PE, high molecular weight polyethylene, is commonly referred to as cathodic protection cable. Like previously mentioned, it is a thick coating around a 7-stranded copper wire which can withstand corrosivity and soil stress.
There are some instances, though, where HMW-PE isn't enough. Sometimes a soil will have brackish water, which will interact with impressed current anodes and create a gas that will corrode the insulation or permeate it, destroying the copper. The copper is integral for getting the direct current to the anode for the sake of the corrosion cycle--if that mechanical connection is lost, then so is the (expensive) anode's purpose.
The thicker solution, then is a dual-jacketed wire. This is the basic HMW-PE cable with another layer. Normally my experience has been with Halar, but another is Kynar. Halar is a polymer that the chloride gases cannot permeate, which protects the integrity of the wire--and thus the deeply buried anodes.
One other use I've heard of for Halar is prairie dogs. Apparently they can chew through the HMW-PE cable, causing breaks in lines. The Halar adds enough resistance that the groundbed remains intact.
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