The investigators aim to prospectively study the feasibility and clinical impact of hyperbaric oxygen therapy in acute hospitalized moderate to severe ulcerative colitis flares as an adjunct to standard medical treatment. Specifically, we will investigate the impact of hyperbaric oxygen therapy on clinical response/remission and serum and mucosal inflammatory markers. The investigators expect that hyperbaric oxygen therapy will improve patient responsiveness to steroids and avoid progression to second line therapy during hospitalization.