World's Largest Underwater Cave Discovered: National Geographic

Further reading: https://news.nationalgeographic.com/2018/01/largest-underwater-cave-system-in-the-world-discovered-in-mexico-spd/

A scuba diver measures the length of Sac Aktun underwater cave system. The Gran Acuifero Maya (GAM), a project dedicated to the study and preservation of the subterranean waters of the Yucatan peninsula, said the 347-km (216-mile) cave was identified after months of exploring a maze of underwater channels. (Herbert Mayrl/Courtesy Gran Acuifero Maya Project (GAM)/Handout via Reuters)