Cayo is a big part of that proof - awesome things come in unique packages -,as is seen in this great article from the Huffington Post.
"Belize has a vast cave system for spelunking, through Mayan archeological sites. One may view mysterious Mayan skulls and eerie skeletons, where Mayan kings made sacrifices at Actun Tunichil Muknal, or ATM caves near San Ignacio. Take an inner tube through the underground rivers in the Cayo district. One can scale a water fall, visit archeological sites, dating as far back as 250 AD, float down a river, and repel 300 feet down into the 'Black Hole.' One can climb steep Mayan temples for 360 degree views of the jungle, all while staying in luxury jungle lodges, complete with spa treatments and scrumptious meals."