Beneath the turquoise waters, hidden coves and sailing routes of the Balearic Islands lies a silent underwater forest that filters the water, stabilises the seabed and helps protect the coastline from storms. In the same depths that shelter millimetre-sized nudibranchs, giants such as bluefin tuna and sperm whales occasionally pass through. Over several days, photographer and diver Rafael Fernández Caballero explored Mallorca and Menorca to document this lesser-known side of the Mediterranean. His aim was to showcase its extraordinary biodiversity while highlighting the changes it is undergoing. Posidonia seagrass, marine reserves and the species that are arriving in, or returning to, these waters help explain what is at stake for the beaches enjoyed by millions each summer.
- The underwater forest that keeps the water clear and protects the coast: Posidonia
- What happens when protection works: the lesson of the El Toro reserve (Mallorca)
- Life offshore: turtles and unexpected encounters
- The encounter that changes everything: a sperm whale calf in a vulnerable sea
- Menorca under the microscope: microfauna and sea caves
- Why it matters to you if you live, invest or holiday on the coast
The underwater forest that keeps the water clear and protects the coast: Posidonia
Posidonia oceanica, a marine plant found only in the Mediterranean, forms vast meadows that function like underwater forests. Its leaves provide shelter and breeding grounds for dozens of species, help maintain crystal-clear waters and protect the coastline against erosion. These meadows also capture and store carbon, playing a vital role in a sea that, despite covering less than 1% of the world's ocean surface, is home to around 17,000 species, between 20% and 30% of which are endemic.
This richness coexists with intense human pressure. Fishing, coastal development, pollution, maritime traffic, the arrival of non-native species and rising sea temperatures are all reshaping marine ecosystems. A Mediterranean-wide analysis estimated that Posidonia meadows have lost around 10% of their known extent over the past 50 years, while the western Mediterranean continues to warm. As a result, any story about today's Mediterranean is also a story about these underwater meadows, which support much of the marine life and many of the beaches visible from the shore.
What happens when protection works: the lesson of the El Toro reserve (Mallorca)
The contrast is striking in areas where effective protection exists. Near the El Toro Marine Reserve, schools of barracuda move through the blue waters, groupers tolerate close encounters that would be unthinkable in heavily fished areas, and seabream populate the rocky seabed. Posidonia carpets the seafloor, creating a vivid green landscape and demonstrating how ecosystems respond when given the chance to recover.
During these dives, a particularly significant species made an appearance: the Mediterranean parrotfish (Sparisoma cretense). Although common in the Canary Islands, it is becoming increasingly frequent in the western Mediterranean, where it was once relatively uncommon. It is not an invasive species, but rather an example of how marine life is adapting to rising sea temperatures. What can be observed today in El Toro may offer a glimpse of the Mediterranean's future.
Life offshore: turtles and unexpected encounters
Far from the coastline, the story changes. With authorisation from Spain's Ministry for Ecological Transition and the Demographic Challenge, the team headed into open waters. What appears to be a vast blue expanse on the surface reveals a very different reality below. Any floating object can become a refuge for marine life. A simple piece of driftwood sheltered a small pompano, while other species gathered around this floating oasis. Above, a cormorant skimmed the sea in search of prey.
One of the most memorable encounters came with a school of large bluefin tuna (Thunnus thynnus), some weighing more than a person, circling the divers. This was no coincidence. The Mediterranean is a principal spawning ground for the eastern population of bluefin tuna, and the waters around the Balearic Islands play a key role during the breeding season.
The expedition also recorded a brief sighting of a mobula ray and the presence of a loggerhead turtle (Caretta caretta). Historically, this species rarely nested in the western European Mediterranean, but nesting activity has increased significantly in recent years. In 2023 alone, 29 nests were documented in Spain as part of the InGeNi-Caretta project.
The encounter that changes everything: a sperm whale calf in a vulnerable sea
The final day of the expedition produced an unforgettable sight: a group of adult sperm whales (Physeter macrocephalus) accompanied by their calves. With the boat's engine turned off and observation protocols respected, one of the calves approached us. The resulting photograph says less about the animal's size than about its future. The Mediterranean subpopulation of sperm whales is classified as Endangered and is estimated to contain fewer than 2,500 mature individuals. Seeing a calf in Balearic waters is a reminder that a new generation is still growing in this sea.
This giant predator feeds mainly on cephalopods and can dive exceptionally deep. Its presence highlights another side of the Mediterranean, far removed from its summer image: a world of deep waters, elusive prey and delicate food chains.
Menorca under the microscope: microfauna and sea caves
In Menorca, an entirely different universe emerges among underwater walls, crevices and caves illuminated by shafts of light. Nudibranchs such as Cratena peregrina, flabellinas and small blennies, including the sphinx blenny, inhabit a mosaic of Posidonia meadows and rocky habitats.
Many of these organisms measure only a few centimetres in length, yet they play an essential role in the marine ecosystem, whether by supporting larger species or helping maintain ecological balance on the seabed.
It is through these small details that the fragility of the ecosystem becomes most apparent. When Posidonia declines or pressure on habitats increases, these smaller species are often among the first to disappear. Everything that depends on them becomes vulnerable too.
Why it matters to you if you live, invest or holiday on the coast
Posidonia is far more than a symbol of Mediterranean diving. It helps maintain water quality, slows coastal erosion and reduces the impact of storms, all factors that directly influence the preservation and enjoyment of beaches.
Marine reserves demonstrate that when human pressure is reduced, large fish populations return and biodiversity improves, benefiting activities such as diving and nature tourism. At the same time, rising sea temperatures are already reshaping species distribution across the western Mediterranean, as illustrated by the growing presence of the Mediterranean parrotfish.
A photograph of a bluefin tuna circling in open water, the tracks of a loggerhead turtle on the sand or a sperm whale calf swimming alongside its pod illustrates both what can be gained through conservation and what can be lost through neglect. In the Balearic Islands, efforts in science, education and environmental protection are already under way. The hidden side of the Mediterranean is closely tied to the future of the archipelago's most treasured beaches and the economy that depends on them.
