Stockholm exists because the land rose.

01 What the ice left behind

The phenomenon is called post-glacial rebound and it is no minor curiosity: at the start of the Holocene the land in this region was rising on the order of two or three metres per century, that is, twenty to thirty millimetres a year. The rate has slowed over time but has not stopped: in the Stockholm area the current uplift is around five millimetres a year.

Five millimetres sounds like nothing. On a human scale it is thirty-five centimetres over a lifetime, and on a historical scale five metres per millennium. In a landscape of shallows, rock at the waterline and shallow channels, five metres is not an adjustment: it is the difference between a navigable strait and an isthmus you can walk across.

02 The day the sea was shut out

West of the city stretches the Mälaren, today a large, branching freshwater lake. It was not always: it was an arm of the Baltic, brackish water connected to the open sea. Rising ground gradually narrowed and lifted its outlet until it cut it off; the last connections to the sea were lost between about 1000 and 1300, and by around the twelfth century the Mälaren was functioning as a lake.

And here is the detail that explains everything: the city was founded in that same period, exactly at the outlet. It is no coincidence. A point where a huge lake drains to the sea through a narrow passage is simultaneously the only place all traffic between the interior and the Baltic must pass, and the easiest spot in the region to control.

03 Fourteen islands and a threshold

Central Stockholm is spread over fourteen main islands linked by 57 bridges, and all of them sit at the point where the fresh water of the Mälaren, to the west, meets the brackish water of the Baltic, to the east. That makes this an unusual sort of city: it is not beside a river or beside the sea, it is on top of a threshold between two different bodies of water.

The difference between the two waters can be seen without knowing any geology, and it is one of this city's best tricks. All it takes is a cold winter: the fresh water on the lake side freezes earlier and more solidly than the brackish water on the sea side. There are winters when people walk across the frozen sheet to the west while a kilometre east the water is still open.

04 Outwards: thirty thousand islands

If post-glacial rebound explains the city, to the east it explains something larger still. Heading out of Stockholm toward the Baltic, the water does not open all at once: it opens gradually, among some thirty thousand islands, islets and rocks spread over about eighty kilometres. It is the third largest archipelago in the world and it is nothing more than the same relief, still half-emerged.

It is worth looking at it with that idea in mind. Many of those islands are backs of polished rock that had not yet emerged a few centuries ago, and many of today's navigable channels will gradually become shallows. The archipelago is not a finished landscape: it is a photograph of a process still under way.

05 What Stockholm leaves you

What stays with you is not the postcard of coloured buildings over water, which is exhausted in an afternoon. It is a sense of time-scale rare in a city: that the ground you walk on is not the ground that was here a thousand years ago, nor the ground that will be here in another thousand, and that the very existence of this place is owed to a movement still happening while you visit.

And a different way of looking at water stays too. There is no shore and interior here: there is an extremely long transition between a lake and a sea, with the city set at the exact point where they touch. Everything else —the bridges, the boats, the quays, the weekend islands— is what people have made of that seam.