Complex sentence: On 16 March 2011, the freighter ran aground on Nightingale Island, spilling tons of heavy fuel oil into the ocean, leaving an oil slick threatening the island's population of rockhopper penguins.
Split into simpler sentences: On 16 March 2011, the freighter ran aground on Nightingale Island, spilling tons of heavy fuel oil into the ocean. The resulting oil slick threatened the island's population of rockhopper penguins.