Question: The 80th Indianapolis 500 was held at an automobile racing circuit that was formerly the home of what?
Context: The 2015 IndyCar Series was the 20th season of the IndyCar Series and the 104th season of American open wheel racing.  Its premier event was the 99th Indianapolis 500, which was held on May 24.  Will Power returned as the reigning champion, while Ryan Hunter-Reay was the defending Indy 500 champion.  Chevrolet entered the season as the reigning Manufacturers' champion.  Indianapolis 500 and the season finale counted for double points. The 84th Indianapolis 500 was held at the Indianapolis Motor Speedway in Speedway, Indiana on Sunday, May 28, 2000.  After four years of an ongoing organizational dispute and "split" in Indy car racing, Chip Ganassi Racing, a CART-based team, became the first team to cross the proverbial "picket line."  They arrived to compete in the Indianapolis 500 as a one-off entry which was sanctioned by the rival IRL.  The Ganassi team of Jimmy Vasser and Juan Pablo Montoya were well received by fans and competitors, and were quickly up to speed with the IRL regulars.  Also making a heralded return to Indy was two-time winner Al Unser, Jr. who had switched full-time to the IRL. The 85th Indianapolis 500 was held at the Indianapolis Motor Speedway in Speedway, Indiana on Sunday, May 27, 2001.  Race rookie Hélio Castroneves, a three-year veteran of the CART series, led the final 52 laps and won his first Indy 500.  Penske Racing swept 1st-2nd with Gil de Ferran the runner-up.  Winning car owner Roger Penske scored his 11th victory at the Indianapolis 500, and his first-ever 1-2 finish in the race.  It was a redemption from the team's previous attempt at Indy (1995) in which both of his cars failed to qualify; subsequently followed by a five-year absence (1996-2000) due to the open wheel "split." The Indianapolis 500 is an automobile race, held annually on the last weekend in May to coincide with Memorial Day.  The race is held at the Indianapolis Motor Speedway, located in Speedway, Indiana.  The Indianapolis 500 is an open-wheel car race and is currently sanctioned by Indy Racing League LLC, and has been run as an IndyCar Series event since 1996.  The Indianapolis 500 is considered one of the most traditional and historical races in the world, and is also considered one of the three most significant motorsport races in the world. The Indianapolis Motor Speedway is an automobile racing circuit located in Speedway, Indiana, (an enclave suburb of Indianapolis) in the United States.  It is the home of the Indianapolis 500 and the Brickyard 400, and formerly the home of the United States Grand Prix.  It is located on the corner of 16th Street and Georgetown Road, approximately 6 mi west of Downtown Indianapolis. The 1996 Indy Racing League, the first in the history of the league, consisted of only three races, as the season concluded in May with the 80th Indianapolis 500.  Walt Disney World Speedway was completed in time to host the first race of the season, and the first ever event of the IRL, and Phoenix International Raceway switched alliances from CART to IRL and hosted the second event of the season.  At the conclusion of the three-race schedule, Scott Sharp and Buzz Calkins ended up tied for first place in the season championship.  With no tiebreaker rule in place, the two drivers were declared co-champions.  Therefore, if the tie "was" broken, Calkins would've been declared the season champion, as he had one win as opposed to Sharp's zero. The 1980 CART PPG Indy Car World Series season was the second in the CART era of U.S. open-wheel racing.  It consisted of twelve races, beginning in Ontario, California on April 13 and concluding in Avondale, Arizona on November 8.  The PPG Indy Car World Series Drivers' Champion and Indianapolis 500 winner was Johnny Rutherford.  Rookie of the Year was Dennis Firestone.  The entire season, including the 64th Indianapolis 500, was to be co-sanctioned by both the USAC and CART under the banner of the Championship Racing League (CRL).  However, USAC withdrew from the arrangement after five races. The 89th Indianapolis 500 was held at the Indianapolis Motor Speedway in Speedway, Indiana on Sunday, May 29, 2005.  It was the premier event of the 2005 IndyCar Series season, and the tenth Indy 500 sanctioned by the Indy Racing League.  Dan Wheldon won the race under a yellow flag.  Wheldon became the first British-born winner since Graham Hill in 1966.  It was the first of two victories for Wheldon in the Indianapolis 500 (the 2011 race being the second).  It was also the first victory for Dallara chassis since 2002. The 86th Indianapolis 500 mi race was held at the Indianapolis Motor Speedway in Speedway, Indiana on Sunday, May 26, 2002.  It was the seventh Indianapolis 500 held as part of the Indy Racing League IndyCar Series schedule, and was part of the 2002 Indy Racing League season. The 80th Indianapolis 500 was held at the Indianapolis Motor Speedway in Speedway, Indiana on Sunday, May 26, 1996.  This was the first Indy 500 contested by the Indy Racing League, under the overall sanctioning umbrella of USAC.  It was the third and final race of the 1996 IRL season.  Buddy Lazier won the race, his first career victory in top-level Indy car competition.
Answer: the United States Grand Prix