Have you ever wondered how the vast ocean is divided into smaller basins? The starting point is geomorphology, the study of the seafloor’s physical shape. Principle 1 of the Ocean Literacy Framework reminds us that the Earth has one big ocean with many features. Within this interconnected ocean system, ocean basins are immense, submerged depressions that make up the foundation of the ocean floor, constantly reshaped by tectonic activity (1). If we draw their boundaries at the continental margins—excluding only the continental shelves—ocean basins cover around 65% of Earth’s surface (2). Within these enormous areas lie features such as mid-ocean ridges, abyssal plains, seamounts, trenches, canyons, and plateaus, all of which contribute to the complexity of the deep-sea landscape. Within the broader ocean basins, smaller bathymetric basins are identified, defined as seafloor depressions of variable extent. These are often outlined by closed bathymetric contours and can be found in regions as deep as the bathypelagic and hadal zones (2). In short, the ocean floor is far from uniform: it is a patchwork of structures that shape how water, nutrients, and life itself circulate through the ocean.
When we look more closely at regional seas, subdivision becomes even more complex. Beyond geomorphology, factors such as biogeography and oceanographic conditions also play a role. In Europe, with its 68,000 km of coastline stretching across the Atlantic and Arctic Oceans, the Baltic, the North Sea, the Mediterranean, and the Black Sea (3), these considerations are particularly visible. For example, the Mediterranean has been subdivided into four regions: the Western Mediterranean, Adriatic, Ionian–Central Mediterranean, and Aegean–Levantine Seas (4). These operational boundaries reflect a consensus built not only on seafloor structure but also on ecological and physical dynamics (4). And because Europe’s seas are shared among many nations, subdivision is also tied to cooperation and governance. The EU has developed macro-regional and sea basin strategies to coordinate sustainable development across borders. These include four Macro-Regional Strategies (Alpine, Danube, Adriatic-Ionian, Baltic Sea), three Sea Basin strategies (Atlantic, Western Mediterranean, Black Sea), and the Greater North Sea Basin Initiative (GNSBI).
This week in Brussels, two major gatherings on regional and ocean cooperation took place. On 24–25 September 2025, the EU Macro-regional and Sea Basin Strategies Days, co-organised by DG REGIO and DG MARE, offered a platform for stakeholders and representatives to explore shared challenges and identify new opportunities for collaboration. Immediately after, on 25–26 September, the All-Atlantic Ocean Research and Innovation Alliance (AAORIA) Forum brought together partners from across the Atlantic to exchange on ocean science and strengthen cooperation from pole to pole.
Take a look at the Map of this week to learn the names and geographical extents of the sea basins of the world. Understanding how ocean and sea basins are defined reminds us that these divisions are more than cartographic lines. They influence ecosystems, shape human activities, and guide international collaboration. With major EU events on sea basin strategies and Atlantic cooperation taking place this September, the spotlight is on how science and policy together can sustain the health and productivity of our shared seas to ensure lasting benefits.
Want to get engaged?
- Stay informed by following updates from the EU Macro-Regional and Sea Basin Strategies Days 2025 and the 2025 All-Atlantic Ocean Research and Innovation Alliance Forum.
- Explore resources, follow events and stay updated on news from the EU Macro-Regional Strategies and Sea Basin Strategies.
The data in the map is provided by EMODnet.
[1] https://education.nationalgeographic.org/resource/basin/
[2] Harris, P.T., Macmillan-Lawler, M. (2018). Origin and Geomorphic Characteristics of Ocean Basins. In: Micallef, A., Krastel, S., Savini, A. (eds) Submarine Geomorphology. Springer Geology. Springer, Cham. https://doi.org/10.1007/978-3-319-57852-1_8
[3] https://oceans-and-fisheries.ec.europa.eu/ocean/sea-basins_en