According to the most popular theory, the Brachiosaurus dinosaur became extinct during the end of the Cretaceous period due to the impact of a meteor on Earth's surface.
Scientists think that during the end of the Cretaceous period, a giant meteorite crashed into the ground, creating a fiery shock wave that took out much of the life on Earth. Most of the dinosaurs, like the Brachiosaurus, may have died from the events caused by this impact. The meteorite could have caused a change in the climate that resulted in an ice age that froze nearly everything. Those that could not survive the cold, such as dinosaurs, may have died as a result.