Divers connected two flooded cave systems and found one of Earth's longest caves

Sistema Sac Actun is a labyrinth of flooded limestone passages beneath Mexico's Yucatán Peninsula, stretching over 380 kilometres, most of it underwater. When divers finally linked it to the neighbouring Sistema Dos Ojos in 2018, it briefly became the longest known underwater cave system on Earth — and inside, they found the remains of a mastodon and one of the oldest human skeletons ever found in the Americas.

— Gran Acuífero Maya Project, Sistema Sac Actun — Yucatán Peninsula, Mexico