As we motored back to El Colony, Dr. Guggenheim stared out at the sea meditatively. Even in this pristine location, we can only imagine what the Caribbean must have been like when the first Europeans saw it, he mused. “Only three percent of sea turtles are left today. But when Columbus arrived on his fourth expedition, his son wrote that they saw turtles in such vast numbers that they covered the sea.’ Man, I would like to have seen that.”

All over La Isla, conversation always returned to Castro, the larger-than-life figure who had been its most notorious (if unwilling) resident, and whose grand plans in the 1970s created its golden age as a tropical youth camp. In one private restaurant, La Casa de Toti, the eponymous owner, known only as Toti, told me how his grandfather was sent to the Model Prison for murder and met Castro there in the 1950s. Along with many other inmates, he was freed a few months after the guerrillas rode into Havana in early 1959. “So my grandfather was always a big fan of Socialism,” he said with a wry laugh.

The air of melancholy left by so many vanished dreams was present even when I decided to climb a remote mountain known as La Loma on my final morning, to get a glimpse of the island’s hard-to-visit south. To get there, I went to meet a farmer-turned-guide Arcadio Castro (no relation), who wore a tattered T-shirt and lived in a marble-floor shack roamed by livestock and piled high with home-bottled mango pulp.

A summer heat wave was descending: By 9 a.m., it was already 95 degrees and 100 percent humidity when Arcadio started hacking the overgrown trail with his rusted machete. Yellow butterflies flitted around us as he fondly remembered the day Fidel Castro visited his high school in 1978. The children were all astonished to see the Maximum Leader bounce a basketball up the stairs to their playground and sink it without effort. (He had been a sports champ in his youth.) In the heyday of the revolution in the 1980s, Arcadio had also served in the Cuban army in Angola and he reminisced about the monstrous bugs he found in Africa. As we climbed, there were glimpses between the palms trees of empty green plains leading to distant mogotes.

By the time we reached the summit, sweat was pouring from us in streams and I was half delirious. The view to the southern nature reserves turned out to be entirely overgrown; Arcadio admitted that he hadn’t taken anyone up here in months. I didn’t mind. It was oddly comforting to see these remote heights being reclaimed by the wild. The island’s man-made hopes have been repeatedly dashed, but hopefully nature will prove resilient.

If You Go

Getting There

There are twice daily flights to the Isle of Youth from Havana on Cubana (cubana.cu), the national airline, but they can book up way in advance. Allow a night in Havana before and after the flight in case of cancellations. New travel restrictions imposed by the Trump administration mean that American citizens traveling as tourists should go through an authorized travel agent to ensure compliance (although travel does not need to be in a group, and 12 other categories of travel, such as educational or professional research, are unaffected). Flights and tours can be booked through an international agency such as Cuba Travel Network (cubatravelnetwork.com) or the United States-based Cuba Educational Travel (cubaeducationaltravel.com). On the ground, the best island agency is Ecotur (53-46-32710; reservas.ij@occ.ecotur.tur.cu). Ocean Doctor is expecting to offer educational tours to the wild southern coast in spring (oceandoctor.org/cuba-travel-program/#upcoming; around $5,000 for seven- to 10-day trips.)

Where to Stay

l Colony (rates start at $63 for a single person with half board, hotelelcolony.com/en/) is a good base for divers but quite removed from other attractions — and being state-run, has wretched food. Far more satisfying is to stay in one of the casas particulares, private guesthouses, in the town of Nueva Gerona and make day trips. I stayed in the quaint and comfortable Villa Gerona, run by the feisty Diami, (53-4631-2962, villagerona.com; rooms starting at $23). Others can be found on Cuba Junky (cuba-junky.com).