NEW YORK, July 18 (Reuters) - The explosion that rocked midtown Manhattan on Wednesday was caused by an underground transformer, police said. "We have a transformer explosion at 41st Street and Lexington Avenue. It happened about 6 p.m. this evening and as of right now unknown on how many injuries," a police spokeswoman said. "We have a building that's a bit shaky, but nothing has collapsed," she said, contradicting a witness report from the scene that a building had collapsed.