In September 2002, Walheim served as an aquanaut on the joint NASA-NOAA expedition (NASA Extreme Environment Mission Operations), an exploration research mission held in Aquarius, the world's only undersea research laboratory, four miles off shore from Key Largo. Walheim and his crewmates spent five days saturation diving from the Aquarius habitat as a space analogue for working and training under extreme environmental conditions. The mission was delayed due to Hurricane Isadore, forcing National Undersea Research Center managers to shorten it to an underwater duration of five days. Then, three days into their underwater mission, the crew members were told that Tropical Storm Lili was headed in their direction and to prepare for an early departure from Aquarius. However, Lili degenerated to the point where it was no longer a threat, so the crew was able to remain the full five days.