Mexico is one of the world’s most biodiverse countries, with arid deserts and steamy jungles, raging coastlines and soaring mountain peaks. And in the Yucatan peninsula, a region home to the Mayan culture for millennia, local environmental activists are working to protect their unique subaquatic ecosystems – the cenotes. Mexico correspondent Alasdair Baverstock took the plunge into one of these natural, fresh-water pools to bring us this special report.