Some office workers like to gauge the day ahead by observing what the mood is on the Mid-Levels escalator. The commuter artery down to Central and back has seen serviced offices open up in the confined streets on either side. But where precisely is Mid-Levels? Apart from being halfway up the hill on the north side of Hong Kong Island, it's difficult to define the exact boundaries. Like Judge Potter Stewart's oft-quoted aphorism about obscenity - 'I know it when I see it' - so the Mid-Levels' borders may be hazy, but there's no denying being able to recognise the well-to-do air of its high-tone, high-rise residential blocks. Perhaps the area's outer limits could be said to be around the Adventist Hospital to the east, and Hong Kong University to the west; much above May Road (the tram's most vertiginous stop) and you are verging into Victoria Peak territory, much below Caine Road and you start to tread on SoHo's toes. Originally Mid-Levels was sprinkled with mansions raised and inhabited by wealthy merchants, set in luxuriant gardens. As Hong Kong's population grew, so these statuesque piles were demolished, to be replaced by ever larger skyscrapers, commanding ever higher prices. Most famously, a five-bedroom flat at 39 Conduit Road was reportedly sold in October 2009 for HK$439 million. As it's difficult to imagine how people got around Hong Kong before the advent of the MTR, it seems inconceivable that Mid-Levels existed before the dawn of the escalator in 1993. It also seems incredible that traders and residents along its route raised a chorus of complaints while construction was under way - howls of anguish that turned into expressions of glee as merchants realised customers were being carried to their very doors, and inhabitants twigged that they could whiz to and from Central without paying a cent. No one has come up with a more attractive name for the Central-Mid-Levels Escalator (though one commentator was moved to describe it as 'a stairway to urban heaven'). Some 800 metres long, and rising 135 metres between Des Voeux Road and Conduit Road, it's rated by Guinness as the longest outdoor covered escalator system on earth. It's similarly appreciatively rated by the 55,000 or so souls who travel up or down or both every day of the year. It's a charismatic thoroughfare too, jinking left and right, sometimes turning itself into a simple walkway, at others ascending steeply, always opening up new vistas. Was the final cost - rather more than HK$115 million, more than double the original estimate - worth it? From sneers of 'white elephant', the escalator has become a much valued workhorse, and an integral part of the Mid-Levels experience.