Arguably the most powerful break on Cape Cod, Marconi Beach at the Cape Cod National Seashore fills with surfers on warm summer weekends — especially when offshore storms stir up the swells. Locals also favor Wellfleet’s Whitecrest Beach, which is nearby. Nantucket surfers tend to congregate at Madaket Beach, which has the advantage of being well off the mainland. A calm day has swells of about 3 feet, and sustained waves up to 16 feet are not uncommon. The main break is left from the offshore sandbar.