Gotta be spines for sure, I remember years back learning just to rock over them at a park that had a 3ft going into a 4ft bowl both ways and thought oh no biggie that shit is easy, went back to my local park which was about 5.5ft for the spine and did that a couple times then hung up and smacked the floor. Didn't touch that spine for a good 3 years or so until a friend roped me into trying it again, slipped out whilst rocking over and bonked my head off the floor.