I was waiting for the ferry, standing at Milsons Point Jetty, a wide wooden staircase leading down into the harbour. As the tide rises and falls, it consumes the steps and releases them, leaving ample leeway for the ferry to rest its gang-plank on whichever tier is just the right height above the water.

The tide was just below the second step. Wave after wave lifted the surface of the harbour and slapped it against the bottom of the wooden planks, spurting water through the gaps between them. Slapping the jetty again and again. Again and again. The wood showed signs of wear, it had been through this day after day.

Inexorable, the harbour is going nowhere, it has nothing else to do. It will beat on this jetty day after day until it is broken, until it is out of its way. Such forces have hewn valleys between mountains, this little structure of wood and steel will be beaten down if the harbour has to wait until the end of civilisation to do it. And what has civilisation been, but a blink in the water's eye, a ripple in history?

Impossible performance art.

Take a city, the size of Sydney. Take Sydney itself. Abandon it, Marie-Celeste style. On the stroke of 6pm, Friday night, everybody puts down their cutlery and walks out. Walks out down the middle of the roads strewn with abandoned cars, their motors still running idle, disturbing nothing as they leave. In an instant, the city as we define it, as a metropolis of human beings, dies. The last person out shuts off the power grid.

Then document the reclamation of the land. Like some Seven-Up series, we return each year to document the ravages of the vermin, the rodents and scavengers that clean up what was abandoned. The larger predators that follow them in and make the city their home (all but the biggest predator of all, of course). Trees escape the narrow allotments they were confined in, and break the asphalt with their roots. Document how the city falls when no-one remains to sustain it.

And, in timelapse, the decay of that step, as the tide slaps against it until it is no more.