Zartan was a wealthy businessman and scientist whose experiments with a unique type of underground radiation had scarred the right side of his face, partially turning it to stone. Due to his vanity, Zartan considered himself ugly and subsequently spent all of his time and money trying to discover the source of the radiation. Ultimately he did when his drilling crews discovered a vast subterranean city where all of the occupants were turned to stone by energy given off by large crystals.
But because the crystals were unstable, the crew's drill caused an explosion. The radiation began seeping directly out of the ground for a brief time. The government, concerned by the illegal drilling, sent Dr. Benton Quest to investigate. He and his team found a stone man, Hard Rock, in the ruins, and Zartan's hopes of studying the crystals were dashed when a two-headed stone creature caused an explosion that buried the city. However the Quests escapes with Hard Rock.
Deciding he could study the man of living rock, Zartan activated a sonic device which awakened Hard Rock many miles away. He followed the signal to Zartan's lair, where Zartan revealed he considered Hard Rock a kindred spirit of sorts. However when Jonny Quest and Hadji Singh, who had followed Hard Rock, fell into the lair, Hard Rock turned against Zartan when his henchman Scorpio tried to kill the children. Enraged, Zartan turned the sonic device up higher, threatening to shatter Hard Rock. In response, Hard Rock brought the entire roof of the lair down, presumably killing both Zartan and Scorpio although Hard Rock himself survived, being made of stone.