Workers in bright protective suits clean up Coogee beach after several Sydney swimming spots, including the famed Bondi beach, were closed by lifeguards as more mysterious black "tar ball" globules washed up along the city's shores.  For days, thousands of black spheres -- ranging from the size of peas to tennis balls -- have appeared on the sand, puzzling locals and tourists alike. Chemical testing has identified them as hydrocarbon-based tar balls, local officials said. Tar balls can form when slicks of spilled oil are battered by wind and waves, mixing to form a sticky seawater emulsion that eventually breaks into smaller pieces or "balls". The source is still unknown. IMAGES AND SOUNDBITES