Dr. Carl Maia was a South American geologist discovered the first scientific evidence of Gill Men in the form of a fossilized amphibian hand when he and his team were excavating a tributary in the upper Amazon. Unable to identify it himself, Maia took it to his old friend Dr. David Reed at the Instituto de Biologia Maritima. Before departing, he left his assistants Luis and Tomas to watch over the camp until his return.
At the Institute, Maia's find won the unanimous support his his colleagues, including Reed, Dr. Mark Williams and Dr. Edwin Thompson to mount a second expedition to dig up the rest of the fossil. Unfortunately, when they returned to the camp they discovered, much to Maia's shock, that not only were they unable to find the rest of he fossil, but something had killed Luis and Tomas while he was gone.
Maia pressed on, though, agreeing with Reed that perhaps over time the chunk of the tributary's rocky shore had fallen into the river and been swept downstream into the part of the river identified by Lucas, captain of the Rita, as the Black Lagoon. Once they arrived there, Maia's contribution to the expedition quickly diminished once it was discovered that the lagoon was inhabited by a living, breathing Gill Man. Still, a good shot with a rifle, Maia remained a valuable member of the group, especially when he and Lucas came to the rescue of Reed and Kay Lawrence, shooting and seemingly killing the Gill Man.
Dr. Maia's fate in the 1977 novelization of Creature From the Black Lagoon has him being killed and eaten by the Gill Man, rather than surviving. His first name has also been changed slightly to Carlos.