Underground explosions Saturday rocked Downtown Indianapolis, injuring three people, damaging businesses and causing evacuations. . . . It was the third time in nine days Downtown has been rocked by underground explosions. . . . Investigators said there is no evidence natural gas caused the blast or that the underground cables had been vandalized. . . . More than 50,000 people are expected in the dome today as the Indianapolis Colts and Denver Broncos meet at 1 p.m. in an American Football Conference wild-card game. More police will be on hand today in the area of the dome to direct traffic to and from the game, authorities said.