Thunder Nationals is the name of a monster truck series run by the USHRA. The series primarily consists of monster truck racing and freestyle in indoor arenas. The events typically have only two parallel sets of cars on an empty concrete floor as a race track. The series was once independent but was combined with the USHRA in the middle of the 1990s. Traditionally, the series featured a core group of independent trucks, such as Virginia Giant, Black Stallion, and Eradicator; however, recent changes to make the events more like the flagship Monster Jam series have included the introduction of Monster Jam regulars, like Grave Digger and Maximum Destruction, to Thunder Nationals events. The typical format of the T-Nats shows is a four contest event consisting of :wheelie competition, straight line racing, donut contest, and ending with free style, the favorite of the majority of casual fans.