(S (S (BASENP (DT The) (NNP Deepwater) (NNP Horizon) (NN oil) (NN spill)) (VP (VP (VBD started) (PP (IN on) (BASENP (NNP April) (NN 20)))) (PP (IN after) (NP (BASENP (DT an) (NN explosion)) (PP (IN on) (BASENP (DT the) (NN rig))))))) (PERIOD .))