The thick twisted (stranded) stuff might be tinned copper. I don't know which would be more unusual -- tinned copper or stranded aluminum. Try scraping it with a knife and see if copper is exposed. You should also look at the other end of the cable (in the circuit breaker box) to confirm where the mystery wire is connected -- ground or neutral. Sounds like it should be grounded, but it may have been connected to the neutral before. If you can find an exposed piece of cable long enough to read the labelling on it, it will tell you exactly what it is.