Sistema Sac Actun (from Spanish and Yucatec Maya meaning "White Cave System") is an underwater cave system situated along the Caribbean coast of the Yucatán Peninsula with passages to the north and west of the village of Tulum. Exploration started from Gran Cenote (20°14′47″N 87°27′51″W / 20.24646°N 87.46403°W / 20.24646; -87.46403 (Gran Cenote)) 5 kilometers (3.1 mi) west of Tulum. The whole of the explored cave system lies within the Municipality of Tulum (state of Quintana Roo).
In early 2007, the underwater cave Sistema Nohoch Nah Chich was connected into and subsumed into Sac Actun making it the longest surveyed underwater cave system in the world for some months. Sac Actun measures 215.4 kilometers (133.8 mi) (after connecting Sistema Aktun Hu with 34 kilometers (21 mi) in January 2011) and is second surpassed by Sistema Ox Bel Ha at 232.8 kilometers (144.7 mi). These two cave systems since early 2007 have been frequently exchanging the title as the longest underwater cave system in the world. Including two connected dry caves (Sistema Yax Muul and Sistema Pierre′s East) makes Sistema Sac Actun with 218.0 kilometers (135.5 mi) the second longest cave in Mexico and the sixth longest worldwide.