This LP helped to reinvent Monk's reputation, showing listeners that Monk was not just a "mad genius" but a structured and well-rounded musician with the ability to lucidly interpret and reinvent the works of another composer.
Monk, considered one of the greatest Jazz pianists of all time, recorded Underground in '67-'68. It's the last recording with the Thelonious Monk Quartet and one of the last albums he made for Columbia.