Is this mystery cable safe?
That's definitely power. Looks like ungrounded split-phase from the colors, possibly 10AWG commonly for dryers or hot water heaters.  Might be cordage. 

That is just as scary as it looks IMO, and I would follow it back to source and decommission it with extreme prejudice. 

As for the cable, shuck it and sell the copper, without a ground wire it's useless even for what it's listed for.