Reuters has a neat gallery of a primary school in China that is built inside a gigantic cave:
Children attend class at the Dongzhong (literally meaning "in cave") primary school at a Miao village in Ziyun county, southwest China's Guizhou province, November 14, 2007. The school is built in a huge, aircraft hanger-sized natural cave, carved out of a mountain over thousands of years by wind, water and seismic shifts.