The band followed "In Rock" up with "Fireball" in 1971, which did show hints of creative progressive rock but was still as heavy as the last album. Just weeks later, the band started working on the next album, which would become "Machine Head". An incident at a Frank Zappa & the Mothers of Invention concert would inspire the band to write their most popular song, "Smoke on the Water" which ended up on the album. "Machine Head", released in 1972, was another success and featured several other popular songs like "Highway Star" and "Space Truckin'".