( ( ( Had ( the asteroid ) ) ( collided ( ( with ( the earth ) ) ( ( just ( five hours ) ) later ) ) ) ) ( , ( ( ( ( the ( planet 's ) ) rotation ) ( would ( have ( caused ( ( St Petersburg ) ( to ( be ( right ( at ( its epicentre ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (SINV (VBD Had) (NP (DT the) (NN asteroid)) (VP (VBN collided) (PP (IN with) (NP (DT the) (NN earth)) (ADVP (NP (RB just) (CD five) (NNS hours)) (RBR later))))) (, ,) (S (NP (NP (DT the) (NN planet) (POS 's)) (NN rotation)) (VP (MD would) (VP (VB have) (VP (VBN caused) (S (NP (NNP St) (NNP Petersburg)) (VP (TO to) (VP (VB be) (ADJP (JJ right) (PP (IN at) (NP (PRP$ its) (NN epicentre))))))))))) (. .)))
Had the asteroid collided with the earth just five hours later, the planet's rotation would have caused St Petersburg to be right at its epicentre.
(ROOT (S (PP (IN In) (NP (DT the) (NN past))) (, ,) (NP (NP (NNS asteroids)) (VP (VBG colliding) (PP (IN with) (NP (NN planet) (NN earth))))) (VP (VBZ has) (VP (VBN caused) (NP (JJ many) (NNS organisms)) (S (VP (TO to) (VP (VB die) (PRT (RP off))))))) (. .)))