In which sport did Mark Calaway make his very successful professional career?

Mark William Calaway (born March 24, 1965), better known by the ring name 'The Undertaker', is an American professional wrestler currently signed to WWE as a free agent.

Widely regarded as one of the greatest professional wrestlers of all time, Calaway began his wrestling career in 1984, working under various gimmicks for 'World Class Championship Wrestling' (WCCW) and other affiliate promotions. After signing for 'World Championship Wrestling' (WCW) in 1989, he enjoyed a brief stint as a mid-card performer named "Mean Mark" Callous, prior to joining the 'World Wrestling Federation' (WWF, now WWE) in 1990.

In WWE, Calaway was rebranded as "The Undertaker", and gained significant mainstream popularity as a horror-themed, macabre entity who employed scare tactics and held links to the supernatural. 'The Undertaker' became one of the most prominent figures of the 'Attitude Era', featuring in various pivotal storylines and matches as WWF sustained a boom period in business in the latter 1990s.

His character transitioned into a biker in the early 2000s, before returning a refined version of his previous gimmick in 2004. This version of the character became renowned for 'The Streak', a series of 21 straight victories at WWE's premier annual event, 'WrestleMania'.