John Hopkins

Divers using rebreathers are taught to change instantly to their open-circuit backup tanks if they sense themselves starting to lose consciousness.

Don has practiced the training drill enough times and taught it to hundreds of students.

But this time it's for real.

Don's mouth closes around his scuba regulator and the gas flows into his lungs.

But then, the world begins to spin.

His vision blurs as he starts to rotate through the water.

There is no knowing which direction he's facing, or which way he's moving.

He isn't aware of this now, but when Don switched to open circuit, a tiny bubble formed in his left inner ear, completely disrupting his sense of balance and causing severe vertigo.

He tumbles through the water, completely disorientated, pinwheeling helplessly in a void.

The sensation is surreal, extreme darkness and extreme brightness flashing through his vision.

With no control over his movements and no sense of direction, the biggest risk is that Don will spin off into the black infinity of the chamber, away from his shot line.

And if that happens, he'll be lost in the chasm, unsure of which way is up or down.

As he twists, something flashes across his eyes.

It's his shot line, briefly glimpsed in the water.

As he comes back around, he takes his chance.

The shot line appears again and he makes a desperate instinctive lunge towards it.

A second later, his fingers close around the weighted rope.

Don hangs on with all he's got.

But the grip doesn't stop his motion and he continues to twist and flail.

It's a crazy, confounding blur.