Brain Groves

Most of the human inhabitants of the substation are never seen walking the halls or in the terminals. Invisible, they work tirelessly, endlessly, and arguably mindlessly, powering and controlling the computers of the substation.

They are never seen walking because they have no bodies to walk with.

At some point in the past, humanity realized they would never build a computer better than the one nature provided them with -- their brains. So they decided to improve on that one instead.

Technology grew at a pace never before or since matched once they created brain-machine interfaces. The first clumbsy innovations were simple neural chipsets that augmented biofeedback, allowing for more complex prosthetics than previously. Eventually, as more of the brain was mapped, they were able to transmit information both ways, allowing some crude VR capabilities.

Then came the networking. The mass of humanity became processing units for the most massive supercomputer it had ever built.

Now, it's fairly commonplace for a person to spend some time in a "brain grove". Their minds sleep while their brains are used for processing power. It's an easy way for many to pass the time and make a buck.

And they don't even need their bodies to do it. For long time ventures, conserving on space and energy, a human brain, encased in its titanium-alloy skull, can be relatively easily and painlessly removed from its body, which is then recycled efficiently through a biovat, the modern compost heap. The brain is then plugged into a receptacle in a vaguely tree-like structure, where it lends itself to whatever computing tasks are at hand. At the end of the period of employment, a new body is formed for the brain, and the person's account accredited with the EC's earned, with interest.

Some people have been plugged in for centuries.