I would add to Grannyknot's commentary on manways that many, perhaps most, of them were once used as wagon roads or were graded for logging railroads. There are hundreds of them, but with each passing year they revert more to nature, and many get precious little human use.
One good example would be the old Deep Creek trail, which involved 15 stream crossings. Unless you knew where it was, most sections cannot be found.
While the Park still contains rattlesnakes and copperheads, they are less numerous than was once the case. I think wild hogs are the explanation. I'm out and about a lot and my brother, an avid hiker, even more so. Between us we've seen exactly one poisonous snake in the last decade (in the Park). Outside the Park I've seen a bunch of them. Difference? Fewer wild hogs outside the Park, where they can be hunted.