First insights into an exceptionally deep blue hole in the Western Caribbean: The Taam ja’ Blue Hole

Coastal karst formations may remain unexplored by the global marine scientific research community. Knowledge provided fishermen coupled with on karstic prompted us to explore geomorphometric and physicochemical features of a submerged blue hole in large tropical estuary Chetumal Bay southeastern coast Yucatán Peninsula, Mexico. Differently from well-known literature cenotes no references are made holes region. Sampling surveying were conducted September 2021 scuba, echosounders, CTD profilers, collection water samples. This study revealed previously unknown maximum depth for Taam ja’ Blue Hole, as it is now named, 274.4 meters below sea level (mbsl), making second deepest known world. The Hole has nearly circular shape at its surface, encompassing an area ~13,690 m 2 . sides steep, slopes >80° that form conic structure covered biofilms, sediments, limestone, gypsum ledges. mouth locates ~4.5-5.0 mbsl, which properties change significantly temperature salinity gradients 4.9°C/m 9.8 PSU/m, respectively. Hydrographic profiles show stratified column inside consisting hypoxic layer (5-20 mbsl), chemocline (50-80 anoxic (>110 mbsl). Evaluation anions cations indicated corresponds mainly diluted seawater.