Geoscience Reference
In-Depth Information
Chapter 4
Under the sea
Hidden world
Water covers 71% of our planet. Only 1% of that is fresh water; 2% is frozen, and the
remaining 97% is salt water in the oceans. It averages more than 4,000 metres deep and
reaches down to a maximum of 11,000 metres. All we can easily see is the surface. Scarcely
any sunlight penetrates deeper than 50 metres, the so-called photic zone. The rest is a cold,
dark world that is alien to us, or at least it was until about 130 years ago.
In 1872, HMS
Challenger
set sail on the first scientific voyage of oceanographic explor-
ation. She visited every ocean and travelled 100,000 kilometres in four years, but depths
could only be taken up by single point soundings with a weight lowered over the side of
the boat. So the pace of oceanography was slow until the development of techniques such
as sonar and sediment coring during the Second World War. During the Cold War, Western
powers needed good maps of the sea floor so they could conceal their own submarines,
and required advanced sonar and arrays of hydrophones to detect Soviet submarines. Today,
ship-mounted and towed sonar scanners have mapped much of the sea floor in considerable
detail. The ocean drilling programme has sampled the underlying rocks in many areas, and
