Over half the Earth's surface is covered by water that's more than 3km deep. Some parts are as deep as 11km. That's seriously deep - about 2km deeper than Mount Everest is high!
Most of the deep sea is unknown territory. In fact, more is known about the surface of the Moon than the deep ocean...Earth's final frontier.
Study of the deep oceans is a relatively young science. The first ocean surveys were conducted in the 1870s, on the Challenger expeditions.
Current research is multi-disciplinary, with scientists from different backgrounds coming together and pooling their discoveries to build up a picture of what is down there at the bottom of the deep, deep sea.
Conditions at such extreme depths are very different to those on land temperatures are lower, pressure is higher, food is more sparsely distributed and it is very, very dark! Any life that exists down there has to be well-adapted to these conditions, which for us would be too harsh to survive.
In 1977, an amazing discovery was made two and a half kilometres below the surface hydrothermal vents. These are chimney-like structures on the sea floor which develop when water seeps into cracks in the Earth's crust, and is spewed out again, super heated and containing a rich cocktail of minerals and toxic compounds.
The discovery was totally unexpected, and even more incredible was the fact that the vents supported a rich ecosystem. With microbes at the bottom of the food chain, the vents also support giant clams, tube worms and shrimp.
The deep sea, and it's remarkable features, are investigated by scientists using mini-submarines and remotely operated vehicles (ROVs) - like the one that was used recently to find the Titanic.
Life in the rest of the deep ocean is a lot less common. There is no plant life at all, because there is no light for photosynthesis.
Most of the life that exists in the deep sea is microbial. Some say the diversity of this ecosystem may be richer than the diversity of rain forests!
Multi-cellular animals of the deep sea have to catch their own food. Many depend on the seasonal fall of phytodetritus - dead phytoplankton which falls to the sea bed seasonally like leaf-fall in the Autumn on land. The deep sea is now known to have other kinds of seasonal change and even weather, such as benthic (sea bed) storms.
As well as increasing knowledge of the biology of the deep sea, scientists are learning more about the natural resources it holds resources like manganese, and deep sea oil and gas.
New technologies, such as highly interactive visual environments, are being employed to explore this possible resource. These are giant displays of 3D images with which scientists can interact to share information and make decisions on the best, most environmentally sound methods of utilising natural resources.
Tony Rice is an expert on deep sea biology, having spent his whole working life in this field. A retired academic (previously at Southampton University) he now divides his time between writing and working as an environmental consultant.
He was author of the book Deep Ocean, written for the Natural History Museum. He is also involved in surveying deep sea life on a consultative basis to determine the environmental risks and benefits associated with exploiting natural resources.
Chris German is a marine geochemist at the Southampton Oceanography Centre at Southampton University. He's a world expert on hydrothermal vents and a veteran of many expeditions to probe the ocean floor.
He's particularly good at assessing where hydrothermal vents may form, looking for them and finding them! Not bad, considering they make up less than 0.001% of the ocean floor.
Chris is an enthusiastic communicator of the science of the deep sea and hydrothermal vents, and recently gave a lecture at the prestigious Royal Institution, as part of its Scientists for the New Century programme.
Kate Humble is a television presenter, fronting programmes such as Holiday, The Essential Guide to Rocks, Top Gear and Rough Science.
Kate is fascinated by the deep sea - an interest that she has developed as a keen scuba diver.
She's not an expert in deep sea exploration, but was keen to participate in the programme, asking questions on behalf of the viewer and adding her excitement and awe of the deep sea.
Jacqui McGlade is a regular on The Next Big Thing, with a broad scientific background spanning marine science, maths and the environment.
She is currently professor of mathematics at University College London. Previously, she was director of the National Environmental Research Council Centre for Coastal and Marine Sciences.
She's particularly interested in working out models that allow us to understand how various factors in ecological systems (such as oceans) interact and change over time.
And your host...Colin Blakemore
Colin is Waynflete Professor of Physiology at Oxford University, and director of the Oxford Centre for Cognitive Neuroscience.
A renowned neuroscientist, he's also a TV and radio regular and an enthusiastic promoter of science.
Deep Sea Biology
Edited by JD Gage & PA Tyler
Cambridge University Press
The Ecology Of Deep-Sea Hydrothermal Vents
Cindy Lee Van Dover
Princeton University Press
Natural History Museum
Beneath Blue Waters: Meetings With Remarkable Deep Sea Creatures
Deborah Kovacs & Kate Madin
Ocean Chemistry & Deep sea Sediments
Open University Course Team