The parts of the brain we understand best are on its surface. Your standard textbook picture of the brain shows its exterior structures and functions - visual cortex (vision), parietal lobe (attention and action planning), motor cortex (movement), temporal lobe (complex vision and audition), and so on.

But if you slice the brain down the middle like an avocado, you'll see another half: subgenual cingulum (function unknown), midcingulum (function unknown), medial parietal (function unknown), retrosplenium (function unknown), and posterior cingulate cortex (function truly unknown).

That last one is the most mysterious so it's my favorite. The posterior cingulate cortex (PCC) has been called the 'dark energy of the brain.' It consumes more calories than any other part of the brain (and the brain itself consumes a lot - 20% of the calories you eat). And we have no idea where that energy goes.

It's the Cheshire Cat of brain areas. If you bring someone into the brain scanner, and have them do a task - any task - it turns off (that is, its neurons stop firing). Then, during that delay between trials, it turns on again, only to turn off when the task restarts.

And that's about all we know for sure.

Often, we can look patients with localized brain lesions and try to guess an area's function based on the deficits it produces. This is how neuroscientists recognized the brain areas critical for producing and understanding speech. Unfortunately, the PCC is a rare target for strokes.

It has been suggestively linked to emotion, , consciousness, attentional control, planning, retrospection... pretty much all the things our minds do. Serious scientists in esteemed journals have argued - with compelling evidence - that it governs ' '. In the absence of any compelling theory, it has been called a 'default mode' area - meaning it is active and consuming large amounts of energy by default when are doing nothing in particular, and deactivated otherwise.

Before neuroscientists discovered the PCC, no theory predicted its existence. It's not hard to see why - it doesn't seem like we need a brain area consumes a lot of energy, and, when we need brain power to do something, it shuts down. When you are doing a task in the lab and it fails to shut down properly, those trials are the ones when you are more likely to make errors on the task.

In the 19th century, men in clubs would sit in their leather chairs with scotch and pipes, and speculate on the source of the Nile. There was no information so there was no shortage of theories. A mountain range larger than the Himalayas, enormous underground springs, primordial caves. Today, the PCC occupies the same role in the brain sciences. Neuroscientists sit around in labs and at conferences and make wild guesses, fumbling around in the dark. Your guess is just as good as ours.

We sometimes act like the brain is fully mapped out, and there is nothing new to see, but there are large swathes where we haven't even gotten past step one.