Exterior shot of Fathom Adonia in port

(8:25 a.m. EDT) – Fathom, the "social impact" cruise line created by Carnival Corporation & PLC in 2015, is on its last Cuba cruise this week; it already underwent its last voluntourism sailing in the Dominican Republic last week.

After the line's sole ship, the 704-passenger Adonia, gets back to Miami, it will make a transatlantic sailing to the United Kingdom, where it will be prepped to return to P&O Cruises, another Carnival Corp. line,.

"The refit will reintroduce P&O Cruises branding," spokesperson Michele Anjel said. "Adonia will spend two days in Portland Dorset for a cosmetic refit and will then sail into Southampton on the morning of June 16 to begin her discovery itineraries."

It's a full-circle journey for the ship, which had retained many old school British touches, even as its Fathom itineraries aimed at something more meaningful and modern.

Carnival Corp. announced Fathom (originally spelled with a small "f") in June 2015. The intent of the line was to offer weekly voluntourism cruises, where passengers would spend up to three days in the Dominican Republic doing social impact activities such as teaching English and planting trees, from Carnival's port of Amber Cove. Sailings began in April 2016.

"Fathom has had a remarkable first year laying the foundation for a pioneering new travel deep experience that hasn't existed in the cruise space or otherwise," Fathom President Tara Russell told Cruise Critic. Nearly 10 percent of Fathom travelers who traveled with the brand made return trips, she said, noting customer satisfaction scores were among the highest in the corporation. "In short, the Fathom concept was very well received."

Many in the industry noted that Adonia's small size might make it ideal for Cuban cruises once travel to the long-off-limits island opened up. Sure enough, in March 2016 – before the line had even begun sailing to the DR -- Fathom became the first American cruise line to receive permission from the Cuban government to sail from Miami to Cuba. Bi-weekly cruises to Cuba, with stops in Havana, Cienfuegoes and Santiago de Cuba, began in May 2016.

Adding Cuba to the mix proved a nice boost for Fathom. Even as fares kept going down for its Dominican Republic cruises, trips to Cuba became a draw. The line did a good job of injecting a dose of self-discovery sessions and workshops from the voluntourism trips with more traditional onboard activities such as lectures, language lessons and salsa dancing. (In fact, we've never been on a cruise ship that had so many opportunities to connect with other passengers, in a meaningful way; we saw it as a perfect cruise to take as a solo traveler).

Yet the Dominican Republic cruises faltered; some sailings were notably less than full. Fathom's issues in the Dominican Republic stemmed not from the programming, Russell said, but in communicating to travel agents and the public what the line was all about. "The Fathom DR experiences have been received with wild response and enthusiasm by those who've sailed with us," she said. "And yet, though continually transformative for travelers, onboard team, and local community and partners; it's been a difficult experience to communicate and sell both for travel agents and through our direct channels."

Even after Adonia heads back to P&O, the Fathom brand will live on. Carnival Corp. has said the name will continue as volunteer shore excursions provided on the company's various cruise lines. Already, passengers can book Fathom shore excursions in Amber Cove, such as the coffee and chocolate learning experiences, paper making and creating gourmet bath products.

Meanwhile, Carnival Corp. has sailings to Cuba announced on Carnival Cruises and Holland America Line.  Russell said Fathom will have a role in those.

"We're currently amidst bringing to life new on-ground experiences in other parts of the Caribbean," she said. "Additionally, our Fathom Travel onboard experiences begin going live in various ships heading to Cuba and next month on two of our sister brands. We are also amidst building a new community engagement vehicle for travelers so travelers can begin to have Fathom Travel experiences in their home communities.

"Fathom is just getting started," Russell said. "The industry will continue to enjoy Fathom Travel experiences, in many new locations, geographies and ways. ... We have written the first chapter, and we're excited about the rapid expansion currently underway."

--By Chris Gray Faust, Senior Editor