Mangroves are amongst the most carbon-dense ecosystems on the Planet. The capacity of mangrove forests to accumulate carbon has been assessed and reported at regional, national and global scales. However, small-scale sampling is still revealing ‘hot spots’ of carbon accumulation. This study reports one of these “hotspots” with one of the largest-recorded carbon stocks in the sinkholes (cenotes) of the Yucatan Peninsula, Mexico. We assessed soil organic carbon (SOC) stocks from deep soils (1 to 6 m) of mangroves associated with cenotes and measured massive amounts of SOC up to 2,792 Mg C ha^-1, the highest value reported in the literature so far. This SOC is primarily derived from highly preserved mangrove roots and has changed little since its deposition, which started over 3,220 years ago (± 30 BP). Most of these cenotes are owned by Mayan communities and are threatened by increased tourism and over-extraction and pollution of groundwater in the region. These hot spots of carbon sequestration, albeit small in area, require adequate protection and could provide valuable financial opportunities through carbon projects.

Sampling was conducted in September 2019 with SCUBA equipment (See Supplementary Video or https://www.youtube.com/watch?v=lGVYxnlkHu8&feature=youtu.be). We sampled two cores at Cenote Jaguar and Yaxchen, and four cores at Casa Cenote.  For each core, we used a 60 mL syringe to take one sample at each of the following depths: 0 –15, 15 – 30, 30 – 50, 50 – 100 cm, and every 100 cm until the bottom of the peat. Soil samples were analysed for soil organic carbon (SOC) and δ¹³C with an elemental analyser coupled with an isotope ratio mass spectrometer (EA-IRMS, Serco System, Griffith University, Brisbane, Australia) with analytical standard deviation < 0.1‰.