Most people picture the ocean as a calm, flat surface concealing a vast, quiet void below. This perception makes it easy to imagine the deep sea as static and disconnected from the world above. In reality, the ocean is a layered and dynamic system, with movement extending from surface waves and tides all the way down to the deepest seafloor. New research published in Ocean Science challenges the idea of a motionless deep sea by revealing that even the most remote parts of the abyssal ocean are in constant motion, with important implications for climate, ecosystems, and global circulation.
The study focuses on the central and eastern Pacific, home to some of the largest abyssal regions on Earth, where the seafloor lies four to six kilometres below the surface. These environments are shaped by vast abyssal plains, fracture zones, and seamounts, and are characterised by darkness, cold temperatures, and immense pressure. Just above the seabed lies the bottom mixed layer, a zone where temperature, salinity, and density are relatively uniform due to continual stirring caused by contact with the seafloor. Rather than being a thin boundary, this layer can extend hundreds of metres upward and plays a critical role in exchanging heat, nutrients, and sediments between the seafloor and the open ocean.
Historically, observations of this deep layer have been rare. Most oceanographic measurements concentrate on the upper few kilometres of the ocean, leaving the deep Pacific sparsely sampled due to cost and logistical challenges. While scientists have long known that cold Antarctic waters flow northward along major seafloor features such as ridges and trenches, the detailed processes by which these flows interact with seafloor topography to stir the bottom mixed layer have remained poorly understood. This lack of data has limited understanding of how the abyssal ocean connects to broader ocean circulation.
To address this gap, the researchers combined new surface-to-seafloor measurements from a trans-Pacific expedition with two decades of high-quality repeat observations. These data enabled them to analyse temperature and pressure profiles across vast regions of the Pacific and apply multiple methods to identify the bottom mixed layer. Machine learning techniques were used to determine which factors best explain variations in its thickness. The results showed striking variability: in some areas the layer was less than 100 metres thick, while in others it exceeded 700 metres.
Crucially, this variability is not random. It is controlled by seafloor depth and by interactions between internal waves generated by surface tides and rugged seabed landscapes. These findings overturn the assumption that the deep sea is stagnant, showing instead that the abyssal ocean is continually stirred by distant surface processes and shaped by underwater topography. Like coastal waters, it has its own drivers of motion, operating over larger distances and longer timescales.
Understanding this dynamic behaviour matters. The bottom mixed layer influences how heat is stored and redistributed in the ocean, affecting long-term climate projections. Oversimplified representations of seabed mixing in climate models can therefore lead to significant errors. The layer also governs sediment transport and deep-sea ecosystems, which is increasingly relevant as interest in deep-sea mining grows. With large areas of the Pacific deep sea still unsampled, the study highlights how limited current knowledge remains. Recognising the active, connected nature of the abyssal ocean is essential for informed decisions about climate, ecosystems, and the future governance of the high seas.