Hydro-Base is a floating seacraft disguised as a natural island floating off the coast of North America outside US territorial waters. Its first known user was the mad ecologist Dr. Herman Frayne, aka Doctor Hydro, who used it both as a laboratory and an airbase on which to land hijacked planes. Doctor Hydro planned to turn the planes’ passengers into amphibious people, using Terrigen Mist he acquired from the renegade Inhuman, Maelstrom. After Doctor Hydro died, his subjects (now known as the Hydro-Men) inhabited the pseudo-island, along with the marine scientist known as Stingray (Dr. Walter Newell). The Hydro-Men lived on the island until Mister Fantastic and Inhuman scientists found a way to cure them. Stingray continued to use the facility as an oceanographic laboratory, and a residence for himself and his wife, who would serve as caretakers. When the FAA revoked the Avengers’s flight privileges from their mansion, Stingray invited them to create a private airfield on Hydro-Base. The Avengers later moved their headquarters to Hydro-Base when their mansion was destroyed by the Masters of Evil, and Stingray became an unofficial member of the Avengers until they returned to their old base. Hydro-Base was destroyed however by Doctor Doom, although Walter and his wife were able to escape.