Lots of Mayan sites have “portals to Xibalba, many of the best are near old sea levels and are now insanely dangerous dive sites, many of those discovered are in Belize. Also many chaltun and other features are natural products of karstic (limestone being desolved) topography and the water levels were a bit lower (late post-glacial sea-levels) so they would have been more of an exercise in wading through bat-filled caves to access, but now make very cool dive sites.