After two decades of living in the Conejo Valley and driving PCH countless times, I've always rushed by the gigantic sand dune located on the east end of Thornhill Broome Beach in Malibu but never actually stopped by to inspect it. With the urging of my 6 year old, we decided to check it out over Labor Day weekend a few years ago.
Driving north up PCH, you'll see the sand dune on the east side after passing by Sycamore Canyon/Sycamore Cove Beach and before the reaching the entrance to the Thornhill Broome Campground. CLICK HERE for more information about various beaches in Malibu.
Park on the shoulder off of PCH. There's plenty of room, though you may be partially parked in the sand. You can also park on the beach side of PCH but rather than taking your life in your hands jaywalking across PCH, you're better off parking on the mountain side of the highway.
The mysteriously large sand dune is the result of "Aeolian transport," or the wind's movement of particles, in this case, sand. Quite steep this hill is, but your effort will be rewarded with some beautiful views of the Pacific Ocean. Plus, you get a nice bird's eye view of Mugu Rock.
We noticed that the southern-most portion of the sand dune seems to have more large rocks in the sand from erosion of the mountains above. The northern hill, at least the (now) several times we've been there, has somewhat less rocks, leaving the sand smoother and more comfortable to walk on.
Coming down the hill is certainly a lot easier than going up!