The Taino knew the island as Amona, and they left their marks deep inside its limestone caves -- petroglyphs and pictographs that archaeologists have studied for decades. Spanish colonists followed, carving their own graffiti into the same cave walls during the colonial and piracy eras, layering one civilization's record atop another. Spain ceded Mona to the United States in 1898 under the Treaty of Paris, along with Puerto Rico, Vieques, and Culebra. By 1919, the island was declared an Insular Forest. The Mona Island Lighthouse, built in 1900, was listed on the National Register of Historic Places in 1981, and the entire island joined the register in 1993. For most of Mona's recorded history, though, the primary draw was not beauty but fertilizer: guano mining dominated the island's economy, and the ruins of that infrastructure -- rail tracks, loading platforms, excavated caves -- still scar the landscape.

The Mona ground iguana is the largest native land animal in the entire Puerto Rican archipelago, stretching 1.22 meters from snout to tail. It exists nowhere else on Earth. Only about 1,500 remain, their numbers driven down by feral cats that prey on the young and wild boar that destroy nesting sites. Although iguanas roam the whole island, they nest in just one small stretch of the southwestern coast -- the only place with loose sand and enough direct sunlight for their eggs. Watching one of these ancient reptiles move across the flat limestone plateau, unhurried and seemingly indifferent to human presence, it is easy to forget that the species is critically endangered. Conservation teams are working to remove invasive predators, but the math remains precarious: one bad hurricane season, one population crash among the juveniles, and the arithmetic could tip.

Mona shelters the largest population of the Puerto Rico applecactus, locally called higo chumbo -- meaning "weighed-down prickly pear" for the way its arms lean under the weight of their own fruit. This critically endangered cactus survives almost exclusively in the Mona Passage islands: 59,000 individuals on Mona, 148 on tiny Monito, and just 9 on Desecheo. The surrounding beaches host the largest nesting site for hawksbill sea turtles in the Caribbean, a distinction that helped earn Mona its designation as critical habitat in 1977. Not everything survived, however. The Puerto Rican parakeet, once common here, went extinct during the first half of the twentieth century. The Puerto Rican amazon parrot disappeared from Mona as well, clinging now to small populations on the main island. These losses make the species that remain feel that much more urgent.

Monito, Mona's smaller companion, rises from the sea three miles to the northwest -- a sheer-cliffed speck less than a mile across, home to the rare Monito gecko and little else. Together the two islands were designated a National Natural Landmark in 1975 and a nature reserve in 1986. BirdLife International recognized them as an Important Bird Area in 2004. Visitors who secure permits can camp, hike, and fish on Mona, and they are even encouraged to hunt the invasive boar that threaten native species. But Mona is not trying to attract crowds. It has no infrastructure beyond what the researchers and DRNA personnel who live there require. The difficulty of arrival is the point -- a filter that ensures only those willing to earn the experience will have it. In the age of overtourism, Mona remains a place where wildness is the amenity.