IT is the world's longest escalator crawling past thousands of backyards and bedrooms, sometimes seemingly only inches away. But what's more phenomenal about the Mid-Levels escalator, which runs from Queen's Road Central to Conduit Road, is that for many people it has unlocked a hidden part of Hong Kong's past. And the ride is absolutely free. Since the escalator went into operation two months ago, even locals have expressed surprise at the wealth of history tucked away in what was the old Chinatown on Hong Kong island. ''If I did not ride on this escalator, I'd never know these old places existed. I'd love to come up again,'' said one 53-year-old. And why not? It takes just over an hour to blaze the trail of the escalator. The moment passengers embark, they are automatically forced to slow down and take note, for the first time, of their surroundings. As the moving stairway inches slowly away from the hustle-bustle of Central, the voyage of discovery at the heart of the concrete jungle begins. No 93 Queen's Road Central, where the old Central food market once stood, marks the beginning of the trail. The present day Central Market is a reconstruction of the old market which was completed in early 1939. It is now one of the rare examples of the early international style of Walter Gropius and the Bauhaus, in vogue in the 20s. But as you get close to the white-washed building you will be hit by an overwhelming odour - the ground floor is still, as in the past, occupied by poultry and fish stalls. The climb becomes steeper on Cochrane Street, where no doubt the escalator passes by too close for the comfort of residents of nearby flats. Here, the incessant drumming and pounding noise of construction has faded into the distance. What immediately catches the eye are the old verandahs, with fancy architecture and railings. Once symbols of wealth, many are still well-preserved in the area. According to local historians, the area below Caine Road was once known as the old Chinatown. Above this boundary line lay the residential area almost exclusive to British expatriates during the early colonial days. Below, buildings inhabited by the Chinese are typically six to eight storeys high, with time-worn exteriors painted in fading colours. Some units are evidently derelict, with wooden shutters tightly closed. Almost all residents living alongside the escalator now keep their blinds shut in a bid for privacy, while office windows are often covered by advertisements seeking secretaries and other clerical staff. The further you move along the escalator, the more peaceful the surrounding area becomes; but new development sites are ubiquitous. Before the escalator reaches Hollywood Road, the grand, greyish blue Central Police Station slowly rises into view. The police headquarters were built alongside Victoria Prison in 1864. Originally it was only a three-storey structure, but an extra level was added in 1905 to cope with the expansion of the police force. Fourteen years later, an extension facing Hollywood Road was built, and it is this part which you can see from the escalator. The four-storey building is classic in style, with the entire facade and the main architectural features at the rear of the building decorated in cement plaster. Other buildings of interest in the area include Victoria Prison on Old Bailey Street and the Central Magistracy on Arbuthnot Road. In Shelley Street the odd old antique shops, Chinese herbal shops and furniture shops can be seen. On Staunton Street you will find the ''Eternal Kindness'' ash repository. Apart from the extraordinary exterior and the sliding iron rod gate, the site is famous because Sun Yat-sen once held meetings there with the Revolutionary Society (Hing Chung Wui)in 1895. A Buddhist nun may be seen quietly folding pieces of sacrificial paper in the darkly lit premises as you pass, and passersby interested in looking around the building are welcome. The next stop on the escalator is Caine Road, the old boundary between the Chinese and British, the subjects and their masters. At ground level in Caine Road, one can steal a peep inside a Chinese tea house where old folk linger most of the morning, drinking tea and chattering. A mixture of aromas - herbal tea, broth and spicy cooking - oozes from nearby kitchens, adding to the Oriental flavour of the area. But what comes next, visually, is a direct contrast to the smell. Having passed the abandoned and overgrown front garden of Leong Fee Terrace, and more construction sites, the tower of Jamia Mosque in pastel green, slowly rises behind high stone walls. But before reaching the mosque, it is worth noting the tongue-twisting name, Rednaxela Terrace - Alexander spelt backwards, an error never redressed. The Jamia Mosque was built in 1850 and was reconstructed on the same site in 1915. The front minaret, the tower from which people are called to pray, is a typical Muslim feature. At certain times, passers-by can hear the incessant murmuring of prayers coming from the mosque which, as those bold enough to trespass would discover, is a tape recording. Robinson Road and Conduit Road mark the end of the escalator, but not the journey. Descending from Mid-Levels at ground level is only the beginning of a close encounter of a cultural kind.