Cheonggyecheon

Where it sits. Cheonggyecheon runs west to east through the middle of downtown Seoul for 10.9 kilometres, from a plaza a short walk south of Gwanghwamun to the point where it joins the Jungnangcheon and then the Han River near Seongdong District. For most of its restored length the water sits four to five metres below street level in a walled channel, so walkers move in a trench beneath the traffic, with twenty-two rebuilt bridges crossing overhead. The stream separates the old administrative city to the north from the commercial and manufacturing quarters to the south, a division that has held since the fourteenth century and still explains the difference between Jongno and Euljiro.

The story behind it. In the Joseon capital this was the drainage line, not an amenity: the seasonal watercourse into which the walled city shed its rainwater and a good deal else. Successive kings dredged it, most famously Yeongjo in 1760, and its bridges — Gwangtonggyo, rebuilt in stone in 1410, and Supyogyo, whose stone gauge measured flood levels from the 1420s — were landmarks of the capital. In the twentieth century the stream became a slum frontage and then a public health problem; it was culverted over in stages from the 1950s, and an elevated expressway was thrown across the top of it between 1967 and 1971. That road was the emblem of Korea's industrial boom and, by the late 1990s, of everything wrong with it: structurally decayed, choked, and sitting on a buried sewer.

The restoration. Demolition of the expressway began in July 2003 and the restored stream opened in September 2005, at a cost above 386 billion won, roughly 281 million US dollars at the time. It is not a natural river. The channel is fed by water pumped up from the Han and by groundwater drawn from subway tunnels, on the order of a hundred thousand tonnes a day, and the banks are engineered stone and planted terraces. Critics called it a fountain rather than a stream and objected to the cost and to the speed of the clearance, which displaced thousands of small traders and roadside stallholders. Two decades on, the arguments have narrowed to design rather than principle: measured summer air temperatures in the corridor run several degrees below the surrounding streets, and the walkway carries a constant flow of office workers, couples and older residents from early morning until late.

On screen and in memory. The pre-restoration version of this place — the elevated deck, the tin-roofed workshops and machine-tool shops underneath, the tangle of Sewoon Sangga to the south — is the Seoul of Korean cinema's breakthrough years, the grimy urban register that Park Chan-wook was working in when Oldboy was shot in 2003, the very months the expressway was coming down. The restored stream now does the opposite job: it turns up constantly in television drama as a place for a night walk and a difficult conversation, lit from below, with the traffic reduced to a hum overhead. The two versions of the same 10.9 kilometres, twenty years apart, are a compact history of how the city wants to be seen.

What you actually see today. Walking from the western end: the Cheonggye Plaza, with a twenty-metre spiralled sculpture by Claes Oldenburg and Coosje van Bruggen installed in 2006 that Seoulites have never entirely warmed to; the rebuilt Gwangtonggyo; a 192-metre ceramic mural depicting King Jeongjo's 1795 royal procession to Hwaseong; stepping stones at intervals for crossing dry-shod; carp, egrets and grey herons in the shallows; and, further east, the Beodeul bridge and a memorial to Jeon Tae-il, the garment worker who set himself alight in November 1970 at the Pyeonghwa Market on this bank in protest at factory conditions, an act that founded the modern Korean labour movement. The Cheonggyecheon Museum stands near the eastern end, opposite the point where the channel finally opens out.

Getting there and timing. Access points are frequent: Gwanghwamun, City Hall, Jonggak, Euljiro 1-ga, Jongno 3-ga, Euljiro 4-ga and Dongdaemun History and Culture Park stations all sit within a couple of minutes of a ramp or staircase. The full walk takes around two and a half hours; the first three kilometres carry the landmarks and the last five are quieter and greener. Ramps exist at intervals but not at every entrance, so step-free access needs planning. Late spring and early autumn are the best months, and summer evenings are genuinely cooler down in the channel than up on the road. During heavy rain the walkway floods by design and is closed at short notice — sirens and gate lights give the warning, and it is not a place to test.