Imagine a mighty, pristine river system. Then put a lid on it and imagine it underground, coursing through yawning, cathedral-sized limestone caverns, pressing gently past prehistoric bones and sustaining an abundance of unique wildlife as it pushes inexorably to the sea and a vital role flushing mangroves and succouring reefs. Seems like a great foundation for one of the world’s fastest growing tourist projects right? Wrong, say environmentalists who fear the tearaway development atop the magnificent Yucatan Aquifer in south-east Mexico will spell its demise.