Introduction

Until the mid-twentieth century, knowledge of the seabed depended on charts using lead-lines and soundings, often many kilometres apart. The ocean was mysterious, the realm of danger, myth and monsters. Today our picture of the underwater world is much better.

Marine charts

Early descriptions depend on what was caught in fishing nets or seen from ships. Occasionally the bodies of large fish or mammals were washed ashore. The rest was mystery: the sea on maps was often embellished with drawings of sea monsters and ships. The earliest charts were drawn to help navigation along the coast. Sailors needed to know where there were anchorages and reefs. Chart-makers drew the land seen from the sea to identify the main landmarks. The earliest charts have very few records of depths.

Charts show most detail around the ports where the risks of running aground were greatest (see for example, Mackenzie’s charts of Weymouth Bay and Poole Harbour). Traditional methods of chart-making were unchanged until the mid-twentieth century.

Undersea sound and its uses

Human activity in the sea makes sounds. Military use of submarine sound is for communication and detection. Seabed mapping depends on sound. Since the Second World War, rapid advances have been made in the use of sound to map the seabed using echo sounders and side-scan sonar. It is now possible to see the seabed in detail.

Nature’s sounds in the sea

Marine organisms have to cope with very limited light, and depend upon sound and touch. The living submarine world communicates by sound. Light transmission is very poor: the human visual range underwater is typically less than 30 m and visibility is less than 5 m. In contrast, sound can travel very large distances, particularly at low frequencies. Marine mammals communicate and locate themselves sonically. Bottlenose and Common Dolphins and Harbour Porpoise make clicks and whistles as they navigate and search for food. Snapping shrimps or mussels make a crackling sound opening and closing their shells.

The beauty of the seabed

Rocky ledges, sand ripples, boulder fields, gravel banks and deep holes in the seabed reflect the way in which the sea has cut into the geological strata and transported sands, gravels and boulders across the seabed. For submarine plants, animals and fish, this is home: a complex, often rugged but sometimes almost featureless, landscape. Nothing living in the sea sees much more than its immediate surroundings, but this is a landscape in which there are complex patterns of behaviour (see Marine Ecology) and where the beauty of the occupants’ surroundings has only recently been displayed. Maerl beds off Handfast Point are formed by the rare coral algae Phymatolithion calcareum and Lithothamnion coralloides. Eelgrass also covers large areas of chalk seabed here. In Lyme Bay, the rocky ledges are home for the Pink Sea Fan, one of a number of Mediterranean-Atlantic species at their most easterly location in the English Channel. At Kimmeridge, the Black-face Blenny and Cranch’s Spider Crab are amongst the rare inhabitants of this underwater world.