If asked to name a region that is poorly known and unmapped, perhaps you would suggest a remote part of Africa, or Antarctica, or the ocean floor. But not, I think, your brain. Although humans have explored the earth and gained insights into far flung reaches of the universe, our brains remain mysterious. Indeed, the human brain has been described as the most complex structure known to mankind. Some basic features are familiar, like the intricately folded surface, the cerebrum with its four lobes, the cerebellum or "little brain", and the limbic system or "emotional brain". Partly from experiments on people who have suffered brain damage, we have some understanding of what happens in these regions. Yet details are scarce, which is unsurprising given each adult brain contains dense networks of nerve fibres, called axons that may have a total length of more than 150,000 kilometres. Now, work is under way on a map showing how the axons are arranged, and detailing the brain's connectivity. The map has been dubbed the connectome, a term coined separately in 2005 by neuroscientists Dr Patric Hagmann at Lausanne University Hospital, Switzerland, and Dr Olaf Sporns at Indiana University, US. The Human Connectome Project was launched by the US National Institute of Health, which in 2010 awarded US$40 million to two consortiums at work on mapping. Much of the money was for sophisticated and new magnetic resonance imaging (MRI) platforms. "The HCP is truly a grand and critical challenge: to map the wiring diagram of the entire, living human brain," said Thomas Insel, director of the National Institute of Mental Health. This would not be the first time a connectome had been mapped, but the only example to date was for a tiny worm with just 302 neurons - nerve cells that typically include axons. By slicing up their bodies and examining them with electron microscopes, researchers produced a detailed wiring diagram. Though the process seems simple, it took around 10 years to create the diagram, and another two decades before a more comprehensive version was published. Like the Human Genome Project, the connectome scheme presents greater challenges. For instance, there are probably 86 billion to 100 billion neurons and 100 trillion synapses. Then, the project focuses on living subjects, rather than slicing through brain tissue. And no one's connectome is fixed, as connections may come and go. Rather than viewing neurons themselves, MRI techniques reveal them by detecting movements of water molecules. Though water does not really flow through brain tissue, individual molecules tend to move faster along alignments of axons - both within and outside them, than they do at angles to them. By combining more powerful magnets than in typical MRI scanners with specially developed analytical techniques, this makes it possible to scan someone's brain and produce a relatively large scale map of neural pathways and connections. Participants and proponents are upbeat about its value. MIT neuroscientist Sebastian Seung is a key player and in his book Connectome: How the Brain's Wiring Makes Us Who We Are , he wrote, "You are more than your genes. You are your connectome." Last month [February], a team published results that support his assertion. They found that differences between individuals were most pronounced in more highly evolved regions of the brain, where the degree of variability in connectivity helped predict how much individuals would differ in behaviour. In a blog post titled "Symphony Inside Your Brain", NIH director Francis Collins predicted that knowledge from the project, "should yield new and better ways to detect, treat, and, ultimately, prevent the brain disorders that now disrupt and devastate so many lives". Some neuroscientists are far less impressed. In an e-mail to a reviewer of Seung's book, Andrew Lumsden, director of the MRC Centre for Developmental Neurobiology, King's College, London, wrote that while "SS's memorable (if pukeworthy) 'I am more than my genome, I am my connectome' may be true in a grossly superficial way, they are not going resolve connectional issues in any useful way using their current approaches." Even so, more work on the brain's structure and workings is under way or planned. There is a Mouse Connectome Project, based in the US. St Thomas' Hospital in London has built a neonatal Magnetic Resonance Imaging Clinical Research Facility that is part of the Human Connectome Project, and will map the brains of babies as they develop in the womb and then after they are born. Martin Williams is a Hong Kong-based photographer and writer specialising in conservation and the environment. He holds a PhD in physical chemistry from Cambridge University.