Harris is a cave diver with over 30 years of experience and dives with the Wetmules team. Harris's cave diving experiences include leading a team of Australian divers to record depths of in 2011, 2012, and 2020 respectively whilst searching for the source of New Zealand's
Pearse River: this mission was filmed for
National Geographic. In 2011, Harris was requested by the
South Australian Police to participate in the recovery of the body of his friend
Agnes Milowka, who had died whilst exploring a cave near
Tantanoola in the south east of
South Australia. Harris is listed as the international regional coordinator for Australia for the International Underwater Cave Rescue and Recovery organization. In 2023, Harris conducted a dive to while using hydrogen as a diluent breathing gas on a rebreather. While hydrogen had previously been tested as a breathing gas, this was the first known case of it being used on a rebreather.
Tham Luang cave rescue In June 2018 Harris was about to depart on a cave diving holiday to the
Nullarbor Plain when he and dive partner
Craig Challen were requested by the Thai government, on the advice of British cave diving experts attempting to rescue twelve Thai children and their soccer coach who were
trapped in the Tham Luang Nang Non cave system, to provide assistance with the rescue efforts. Harris' efforts throughout the rescue have been described as essential; he conducted a medical assessment of all of the trapped boys. At the 18 July press conference, it was revealed that the soccer team decided as a group that the boys who lived the farthest away should leave first, so they could ride their bikes home. To allow the rescue to occur, Harris developed a plan to keep the boys anaesthetised with
ketamine while spontaneously breathing through full face masks. This was to ensure they did not panic during the long extrication through underwater caves, which would have endangered both the rescue scuba divers and the boys. While one source states that Harris was the last rescuer to leave the cave, this is not correct according to Harris's own detailed account of the rescue. On 5 November 2019 Harris and Challen released the book
Against All Odds chronicling their participation in the rescue of the boys from the Tham Luang Cave. In the book they corrected some of the inaccuracies in the media regarding the rescue. Harris stated that he did not pick the order that the boys would leave the cave, and that the boys decided that amongst themselves based on the distance each boy lived from the cave, as they thought they had to cycle back home. He also said the boys were fully unconscious as they were carried through the flooded cave, as he had given each boy two intramuscular injections in the thigh:
ketamine to sedate them, and
atropine to suppress saliva production to stop choking. ==Lieutenant Governor of South Australia==