Located a short speedboat ride from Punta Mita and Puerto Vallarta, the small archipelago of Islas Marietas was designated a national park in 2005 and a UNESCO Biosphere Reserve in 2008. Nonetheless, overtourism damaged the site. The otherworldly Playa Escondida, the “Hidden Beach” where everyone wanted a selfie, suffered in particular, with upward of 2,000 visitors a day leaving trash and damaging the coral reef. Visitor numbers are now carefully restricted. Only certain tour operators can visit the archipelago at all, and a mere 116 people per day can visit the Hidden Beach, which is closed entirely on Monday and Tuesday.

Imanta Punta de Mita booked a private excursion for us, including coveted permits to the Hidden Beach. A guide and captain affiliated with the Mictlan Surf School took us out in a small but speedy boat. En route, we paused to watch a juvenile humpback and its mother. The former breached five or six times. The craggy islands of Marietas are a bird sanctuary, and we spotted pelicans and blue-footed boobies.

After showing our bracelets to the park rangers, we donned life preservers and helmets. To reach the Hidden Beach, it’s necessary to swim through a rocky tunnel at low tide. Fins aren’t allowed, making the trip inadvisable for weak swimmers (even in relatively calm seas, a current is detectable). We emerged into an extraordinary crater fringed with agave. The Hidden Beach forms the soft, sandy bottom of the crater and is lapped by turquoise waters flowing through the tunnel. It was a vision of a tropical paradise, an impossibly beautiful travel poster come to life. And we had the place entirely to ourselves.

Visits are limited to 30 minutes, and our time was up all too quickly. We intended to snorkel nearby, but the water proved too murky. Instead, we circumnavigated the islands’ cliffs, indented with caves and punctuated by rock arches. On the smaller of the two larger islands, we found additional sugary beaches, surrounded by scenic rock formations. We swam ashore to explore them. Even without the snorkeling, the excursion was nothing short of breathtaking.