Into the Mediterranean

In the morning of May 27 2025 Statsraad Lehmkuhl sailed into The Mediterranean Sea. The ship was not alone, 300 ships cross The Strait of Gibraltar daily, roughly one every five minutes.

A busy waterway as the map above show, and once - The Mediterranean was the most important of all seas.

In antiquity, from 700 BCE to 500 CE, it was across the Mediterranean that trade goods, culture, and knowledge were exchanged among the surrounding peoples, Egyptians, Greeks, Romans, Arabs, Persians, and Semites. They called the sea “the great green,” “the white sea,” “our sea,” or simply “the sea.” The name Mediterranean - meaning “the sea in the middle of the world” - came into use in the 7th century.

The seven seas

Have you sailed all seven seas? Then you've sailed the Mediterranean.

“The Seven Seas” once referred to the bodies of water crossed by Greek merchant ships: the Adriatic Sea, the Arabian Sea, the Black Sea, the Caspian Sea, the Persian Gulf, the Red Sea - and the Mediterranean.

Today, we use “The Seven Seas” as a general term for all the oceans on Earth. Modern hydrographers recognize just five: the Atlantic, Indian, Arctic, Pacific, and Southern Oceans. The Mediterranean is considered a part of the Atlantic.

A narrow passage

The Mediterranean is almost completely surrounded by land. The Strait of Gibraltar, which connects it to the Atlantic, is just 14.2 kilometers wide, a 30-minute ferry ride. The reason no bridge has been built is that the strait is between 250 and 900 meters deep and has strong currents.

Plans for a railway tunnel under the Strait of Gibraltar have been in the works for years, and in 2023, Morocco and Spain agreed to begin construction in 2030.

Whether the tunnel will actually be built remains uncertain. The Mediterranean lies in a geologically active area where earthquakes are not uncommon. The Earth’s continents are constantly shifting, and beneath the Mediterranean, the African-Arabian tectonic plate in the south is being pushed under the Eurasian plate to the north.

Long time ago, this slow collision gradually reduced the size of the ancient sea and closed its outlet to the Atlantic. As rivers brought in less water than what evaporated the Mediterranean dried up, and around six million years ago it was reduced to saltwater lakes nearly devoid of life. Today, large salt deposits beneath the seafloor are remnants of this period.

The Zanclean flood

Then, 5.3 million years ago, something dramatic happened.

The Atlantic suddenly broke through at what is now the Strait of Gibraltar, and 100 million cubic meters of water per second surged in at over 140 kilometers per hour. That’s a thousand times more water than the Amazon River discharges into the sea.

This event is known as the Zanclean Flood.

In less than two years, the Mediterranean was completely refilled. The new sea impacted the climate not just locally but globally, and dramatically changed the environment in the region. Animals could no longer migrate between Europe and Africa, which is why the wildlife in southern Spain differs from that in northern Morocco, despite being only a few kilometers apart.

The flood also left animals and plants isolated on islands in the Mediterranean, where they evolved into species found nowhere else, what we call endemic species.

Whales, fish, and other marine life established themselves and adapted to the new sea. Of the 700 fish species in the Mediterranean, about 70 are endemic.

Two currents

There’s still a steady flow of water through the Strait of Gibraltar. Mostly from the Atlantic into the Mediterranean, as evaporation here exceeds the amount of water supplied by rivers. This evaporation makes the Mediterranean water heavier than Atlantic water, so it flows back out underneath the incoming current.

The two currents influence each other, creating special wave patterns deep below where the water masses meet. These internal waves can sometimes be seen in satellite images and are among the phenomena that ESA and the Nansen Center aim to study.

A new opening

The 193-kilometer-long Suez Canal opened in 1869, linking the Mediterranean to the saltier and less nutrient-rich Red Sea. In winter, water flows northward through the canal; in summer, southward.

Plankton, seaweeds, invertebrates, and fish from the Red Sea are now colonizing the eastern Mediterranean, and nearly 100 new fish species have arrived since the canal was opened.

Not so clear anymore

The water in the Mediterranean is relatively low in nutrients and algae, making it appear clear and inviting, but appearances can be deceiving. The sea is polluted with sewage, industrial waste, and ship emissions, and several species are at risk of extinction. One of them is the Mediterranean monk seal, with fewer than 600 individuals remaining.

Several fish stocks are also under pressure, mainly due to overfishing.

Powerful winds

The Mediterranean climate is subtropical, with hot, dry summers and most (but not much) rainfall in winter. It’s perfect for sailing, but the weather can change quickly.

When high pressure builds in the western Mediterranean and low pressure in the east, strong winds can suddenly blow from the north. In Greece, this wind is called the meltemi, most common between June and September. In France, a similar wind is called the mistral, usually blowing in winter and spring.

In the Adriatic, especially off Croatia, another sudden northerly wind occurs in winter and spring. Cold air builds up in the mountains, then rushes down toward the sea. This is called the bora.

Scirocco is a southerly wind that brings hot, dry air from the Sahara northward. As it passes over the sea, it gathers moisture, leading to heavy rain when it reaches land. If the wind carries dust and sand, the rain may be tinted reddish brown—sometimes referred to as blood rain.

Facts: The Mediterranean Sea covers 2,500,000 km², making up about 0.7% of the world’s ocean surface. Its average depth is 1,500 meters, and the deepest point is Calypso Deep, at 5,267 meters, west of Greece.