Originally built as a floating oceanography research station disguised to appear as a natural island in the Atlantic Ocean. It was first used by Dr. Herman Frayne, a.k.a. Doctor Hydro, an insane ecologist. Frayne employed it as a lab and airbase from which to deploy hijacked aircraft. Doctor Hydro wanted to transform the hijacked passengers of five jets into amphibious merpeople. The active ingredient in his transformation catalyst was the Inhumans' mutagenic Terrigen Mist, which he acquired from the renegade Inhuman hybrid Maelstrom.
Following Frayne's death, the five-hundred victims of his experiment, known as the Hydro-Men, stayed in 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. 
After that time, Stingray used the facility as an oceanographic laboratory and as a residence for himself and his wife Diane. When the U.S. Federal Aeronautical Association revoked the Avengers' flight privileges from their midtown Manhattan mansion, Dr. Newell invited the group to lease part of the island to create a private airfield on Hydro-Base, which was located only nine miles offshore from New York. The Avengers accepted his offer, relocating their supersonic quinjets to the floating island and constructing the UGABS, or Underground Airbase Shuttle, as a rapid means of transport to it from Avengers Mansion. The Fantastic Four also moved their larger vehicles to the island at that time. As an Avengers outpost, Hydro-Base offered its airliner-capable runway, a Research and Development facility including several robots, and a quinjet hangar.
Many years later, the Hydrobase was retrofitted, repurposed, and relocated to the Pacific Ocean by Sunspot as the headquarters of his new Avengers Idea Mechanics. This location served as the main base of A.I.M. as well as a decoy for Avenger Base Two, the headquarters where "the real work" got done.
When S.H.I.E.L.D. attacked the Avengers Island in retaliation for a war crime, Sunspot was forced to abandon the location and move all of A.I.M.'s operations and operatives to Avengers Two.
This complex was originally designed as an operations facility for S.H.I.E.L.D. by Stark Industries, and later hosted the Avengers headquarters.
Seeking them out, Loki Laufeyson peered inside the building to see what his old foes were doing.
Thor's strength was tested out here by Henry Pym to find his upper limits.
- Joseph Jennings
- Namor McKenzie
- Tamara Kurtz
- Avengers Idea Mechanics
- Hydrobase's creator/designer could be the Imperial Japanese scientist who created an artificial island during World War II which housed Shaggy Men.
- 116 appearance(s) of Hydrobase
- 3 minor appearance(s) of Hydrobase
- 14 mention(s) of Hydrobase
- 5 image(s) of Hydrobase