If you have any conductive bar or wire of sufficient size you can safely touch both ends (and measure zero volts across them. If you stretch those conductors over miles and miles of distance, it gets harder to reference the end to both grounds at the same time. And getting a good ground can be hard. There are numerous stories of "stray voltage" making cattle jumpy, etc.

Of course if you coil up that conductor and pass a magnet by it you're going to get shocked ;)