Cueva de los Tayos in Ecuador and vast Mexican systems like Sotano de las Golondrinas represent the frontier of deep-cave exploration, blending rigorous scientific mapping with enduring legends of hidden treasures. Influential figures like Juan Moricz and Stan Hall fueled theories of a subterranean "Metal Library", yet major scientific expeditions involving Neil Armstrong concluded that the cave's rectangular passages were natural limestone formations. The legacy of these sites persists through the controversy of Father Crespi’s missing golden artifacts and ongoing expeditions into the Amazon, where geological architecture is often reinterpreted through the lens of alternative archaeological and ancient astronaut theories.