This is part of the Original Muscle Beach in Santa Monica, next to the Traveling Rings. There are several poles in the sand that stick out 6-feet high that allow for 20ft to 100+ft long slacklines, tricklines and recently there have been two long-line poles that are 10-feet tall that allow for 260-300ft longlines.
(Note: This is NOT at the Venice Muscle Beach, but at the Original Muscle Beach in Santa Monica.)