At least 24 people were killed and 271 others injured when several underground gas explosions ripped through Taiwan's second-largest city overnight, hurling concrete through the air and blasting long trenches in the streets, authorities said today. The series of explosions about midnight Thursday and early today struck a district where several petrochemical plants operate pipelines alongside the sewer system in Kaohsiung, a southwestern port with 2.8 million people. The fires were believed caused by a leak of propene, a petrochemical material not intended for public use, but the source of the gas was not immediately clear, officials said. Video from a TVBS broadcast showed residents searching for victims in shattered storefronts and rescuers pulling injured people from the rubble of a road and placing them on stretchers while passers-by helped other victims on a sidewalk. Four firefighters were among the 24 dead.