Looking for a fun way to explore one of the most popular tourist destinations in Arizona? Celebrate Earth Day and join Run Tucson on Saturday morning, April 19, 2025 for the TMC Earth Day 5k Run/Walk entirely on the grounds of the world-famous Biosphere 2. Run the Desert . . . then visit a rain forest, view an ocean reef, and wander through savanna grass lands . . . all in one day!