Millennium Bridge, London

Where it sits. The Millennium Bridge crosses the Thames between Peter's Hill on the north bank, directly below the south face of St Paul's Cathedral, and Bankside on the south, where it lands at the doorstep of Tate Modern. It is a pedestrian bridge only, with no road deck and no vehicles, and the alignment was chosen so that walkers coming north see Christopher Wren's dome framed dead centre and rising as they climb the steps. It was the first new crossing in central London since Tower Bridge opened in 1894.

The engineering, and the wobble. The bridge is the work of Foster and Partners with the engineers Arup and the sculptor Anthony Caro, and the design brief was for a blade of light: a suspension bridge whose cables are pulled so taut that they sag only about 2.3 metres over a 144-metre central span, roughly six times shallower than a conventional suspension bridge. The total length is around 325 metres in three spans, with a four-metre aluminium deck carried on steel arms outside the cables. It opened on 10 June 2000 and closed two days later. Tens of thousands of people crossing at once had triggered synchronous lateral excitation: a small sideways sway makes walkers adjust their footing in step, which feeds the sway, which recruits more walkers. The fix took nearly two years and cost about five million pounds — 37 fluid-viscous dampers to absorb lateral movement and 52 tuned mass dampers to handle vertical bounce, most of them tucked under the deck where you can see them from the riverside walkways. It reopened on 22 February 2002 and has been steady ever since, though the nickname stuck.

On screen. The opening sequence of Harry Potter and the Half-Blood Prince destroys it. Death Eaters, rendered as ribbons of black smoke, tear down the Thames past Tower Bridge and the City, wrap the Millennium Bridge, twist its deck through ninety degrees and drop it into the river with people still on it. It is the film's declaration that the war has crossed into the Muggle world, and it was built as a visual effects sequence using plates shot on and around the bridge and the surrounding riverbanks rather than any physical destruction. Readers of the novel will notice the change of geography: in the book the bridge that collapses is the fictional Brockdale Bridge, somewhere in the countryside, reported second-hand in a conversation between the Prime Minister and Cornelius Fudge. Moving the catastrophe to a real, recognisable London landmark was a screenwriting decision, and it works precisely because the audience knows the bridge is standing.

What you actually see today. The crossing takes three or four minutes at a walk. The best of it is the northbound approach: from the Bankside end the dome of St Paul's sits directly ahead, and the steps and the rising deck reveal the west towers as you climb. Look down at the aluminium deck and you will find hundreds of tiny paintings on flattened chewing gum, an unofficial and long-running work by the artist Ben Wilson. On the downstream side the view runs to the Shard, Tower Bridge and the City cluster; upstream you get Blackfriars station built out across the river on the piers of the old railway bridge. The bridge is lit from within the handrails at night, which is where the blade of light idea finally reads properly.

Getting there and timing. St Paul's Underground station is five minutes from the north end, Blackfriars and Mansion House about ten, and Southwark or London Bridge serve the south side. There is no charge and no closing time. Crowds peak from late morning to late afternoon at weekends, when the bridge is a slow shuffle; early morning, especially before eight, gives you an almost empty deck and low light on the cathedral. Sunset from the middle of the span, looking west past Blackfriars, is the classic photograph, and blue hour on a clear evening is better still.

Making a route of it. Tate Modern, free to enter, sits at the southern landing in Giles Gilbert Scott's former Bankside Power Station; Shakespeare's Globe is two minutes east along the river; the Founders Arms and the Bankside terrace face back at St Paul's. On the north side, the climb up Peter's Hill brings you to the cathedral steps in three minutes, and the Thames Path leads east under Southwark Bridge towards Borough Market, which is itself a Potter location.