As a teenager I used to like going to the museums at South Kensington on my own.

This might sound like an odd thing to want to do, but what with the cheap tube tickets (half-fare was for a time set at a flat rate of 10p even if you wanted to go to Ongar) and the fact that the museums had no entry fee it was a cheap way of entertaining myself.

As a younger child these visits with adults had always been a special treat but had been slightly frustrating as you were subject to the whim of your guardian (whether parent or teacher) with regard to how long you could spend staring at things. This could get annoying if you wanted to try one of the demos - more often than not there would be a scrum of five year olds around the button jabbing it pointlessly and not waiting to see whether a tinny voice would start to emerge from a speaker or the intricate brass mechanism within the display case would start to operate with slow inevitability. By the time it became free you'd have been dragged off into the next chamber. I longed to have the place to myself, like the Time Traveller in H G Wells's The Time Machine when he discovers the Palace of Green Porcelain in the year 802,701.

Once I was old enough to travel solo on the tube, going to South Kensington on my own at weekends or during the summer holidays was the nearest I could get to this ideal.

The excitement started when disembarking at South Kensington. Rather than making my way up to street level I always used the "Foot Tunnel to Museums" - a long, wide Victorian subterranean passageway lined with ceramic tiles and with shafts up to pavement level to let in light. Halfway along a set of steps emerged at the corner of the gardens in which the Natural History Museum was set.

I don't know which museum I liked the most; I think it depended what kind of mood I was in. Sometimes it was definitely the Natural History. The complex gothic exterior gave some indication of the myriad wonders concealed within, and my heart always started beating a little faster as I walked through the doors into the Central Hall.

Originally the centrepiece of the hall had been a display consisting of some replica elephants and rhinoceroses, a display my parents remembered from their own childhood visits. If you wanted to see the dinosaurs - as all right thinking children would - you had to go looking for them, eventually finding the gallery somewhere deep in the museum. Your discovery was made all the more exciting by the fact you'd had to search.

Then they changed it. They moved the diplodocus skeleton into the Central Hall. I didn't like this. Of course I liked the diplodocus itself, it was one of my favourite exhibits (even after I discovered that it was only a plaster replica of an original fossil), but moving it to the entrance like this felt like a sellout, showing people one of the most popular and exciting of the museum's inhabitants the moment they arrived. It was like revealing the climax of a story on page one.

Every time I visited I told myself I was going to look at the whole of the museum and every time I got distracted by something which would take up all my time until I had to leave.

Some of the galleries with things in glass cases were virtually empty the whole time. With a whole cavernous hall to myself, I used to walk up and down the rows staring at the ancient specimens. The fact that they had probably once been collected by long dead Victorian naturalists was almost as interesting as the exhibits themselves. Tucked away in the gloom in one corner were the remains of the last dodo. In other corners flocks of butterflies were frozen under glass, their colours muted in the half-light.

More popular was the human biology gallery, a space age interactive experience on the ground floor that was all curved white surfaces and cathode ray tubes shining in the darkness. It was completely unlike the rest of the museum, like a trans dimensional alien craft that had inexplicably materialised in the cathedral like interior of the old building.

At the entrance stood a group of naked human effigies in white - the nudity robbed of its embarrassment value by the fact that they appeared more like statuary than flesh and blood. Nevertheless, after a few visits I noticed that their bottoms had started to get a bit grubby. I concluded that when confronted with a pair of (plastic) buttocks most tourists found themselves unable to resist having a quick feel.

There was something organic and dreamlike about this gallery. Behind the hubbub of visitors you could hear the subtle sound of a heartbeat - not actually an ambient effect designed for the whole exhibition, but unavoidably leaking out of one of the chambers - an oversize reproduction of a womb into which visitors could walk and contemplate a model of a gargantuan human foetus whist listening to the soundtrack of pre-birth.

Other exhibits in this gallery were interactive. I clearly remember one in a darkened section all about the human brain. In front of a screen was a sloped metal surface from which protruded two levers - comedy archetypes consisting of a metal stick with a plastic ball on the end. Above them was a notice that read "pull one of these levers". I did so and almost immediately a voice boomed out of the darkness.

"What have you just done?"

Despite the accusatory nature of this sudden question, the voice was  calm and reassuring. It then went on to explain the sequence of events that had just occurred - from the eye seeing the words to the brain decoding them and what happened next ("You then realised that you had a choice...").

The sound of this and other exhibits added to the phantasmagorical atmosphere of the  Human Biology gallery. One showed a short film about the fight or flight response in which a man was surprised by a cat in the middle of the night. The man's gasp of fear, his elevated heartbeat and breathing plus the cat's miaow all added to the bizarre ambience as I walked beneath giant red blood cells and examined displays demonstrating the wiring of the human brain and how perception worked. Skeletons stood like sentinels in the corner, adding vestiges of my childhood fear of them to the mix. Strange questions ("duck or rabbit?") decorated the walls and further white mannequins with perspex chests revealing their internal organs stood on pedestals. Models of the cortical homunculus stood in glass cases, their giant hands on pencil thin wrists somehow reminding me of the nightmares I had when delirious.

After this psychedelic experience the calm of the rest of the museum was required simply to ground me. The stuffed animals in painted dioramas were reassuring in their blandness. I mounted the stairs in the Central Hall and made my way to the upper levels, back to the glass cases, seeking out rooms I'd never visited before. On these upper levels I discovered the whale gallery, a full size model of a Blue Whale hanging from the ceiling like the hull of a gigantic airship. Up here in the rafters large skylights let in the sunlight and I felt a long way from ground level. There was so much to see and I could spend all day in here. Nevertheless after a while I would start to feel the compulsion and made my way back to the ground floor and headed towards the rear of the museum.

Natural History was all very well but there was something else I found just as fascinating. Science and technology. My visits to South Kensington were never complete without a visit to the Science Museum. And I knew a secret.

It wasn't that much of a secret really, but in the ground floor tucked away behind a room containing stuffed crocodiles and ostriches was a long, low gallery, the walls of which were lined with glass cases. This passageway led from the Natural History Museum to the Science Museum, and passing under the open shutters at one end you would step out of the world of the organic and were now surrounded by machines, from steam engines to space capsules.

This was another world altogether.