Hydro-Base is a floating sea craft disguised as a natural island, which floats off the coast of North America outside the U.S. territorial waters of the Atlantic. It is not known who designed of constructed the artificial island. Its first known user was the mad ecologist Dr. Herman Frayne, who utilized it as a laboratory and airbase upon which to land hijacked planes. Dr. Hydro was involved in a scientific project to transform the hijacked passengers of five jets into amphibious mer-people. The active ingredient in his transformation catalyst was the Inhumans' mutagenic Terrigen Mist, which he acquired from the renegade Inhuman hybrid Maelstrom. It is possible that Maelstrom had something to do with the island's construction. Following Frayne's death, the five hundred victims of his experiment (known as the Hydro-Men) set up residence on the island base, along with marine scientist Dr. Walter Newell, alias Stingray. The Hydro-Men lived there for a few years until Dr. Reed Richards of the Fantastic Four, working in conjunction with Inhuman scientists, discovered a way to cure the Hydro-Men. Since that time, Dr. Newell has used the facility as an oceanographically laboratory and as a residence for himself and his wife Diane. When the Federal Aeronautical Association revoked the Avengers' flight privileges from their midtown Manhattan mansion, Dr. Newell invited the group to create a private airfield on Hydro-Base. The Avengers accepted his offer, and has since relocated their supersonic quinjets to the floating island and have constructed a special underground shuttle to it.

First Appearance: SUB-MARINER #61