Steel yourself: Richard Serra's monumental sculptures

Richard Serra's gigantic walk-in sculptures, now at the Gagosian Gallery, are monumental in both size and impact. Michael Glover emerges a changed man

The American sculptor Richard Serra, who will be 70 next year, has been wrestling with steel since he was 17. In those days, he was working in the steel mills of California, his home state. When he began to produce work of his own, he was classified as a minimalist. Not any more. As the years have gone by, Serra's brutishly engaging steel sculptures have grown bigger and bigger.

The three large pieces that stand at the centre of his new show at the vast Gagosian Gallery in London must have been as easy to shift into their present locations as rusting steamships. Once, a man died when a Serra fell on him. The shapes of the first two bring together the idea of the doughnut and the idea of the sphere – the gallery list describes this as a pair of "torqued toruses". But we don't need to go into the intricacy of their making to feel some response to these works in our pulses. A child will respond, and immediately, with shock, awe and delight, to what Serra is doing. They look a little like giant, ferociously dangerous, vertigo-inducing, splayed-lipped cauldrons.

Walk inside – yes, much of the pleasure resides in the fact that you can enter Serra's works and wander at your will – and you see that these vast, soaring, enclosing walls of gently oxidising Corten steel (today they look a warm and almost furry, if not velvety, terracotta-ish brown) are leaving oval shapes on the ground. But it is those walls themselves, smooth enough, but also mysteriously streaked and stained, that are so enthralling and disturbing in just about equal measure.

The walls practically scrape the ceiling of this gallery – you almost feel you can see the scratch marks where they didn't quite make it, as sweating gangs of labouring men heaved and strained them into place. But how exactly – in what particular direction – are they listing and leaning? The fact is that we are never quite sure, because our perception of these shapes seems to change as we walk around and through them.

Sometimes, the walls seem to be leaning out, and then, just moments later, they seem to be leaning in. Quite how we should describe these shapes seems to be fairly open-ended, too – do they resemble a cross between a giant ship's funnel and a giant ship's prow? Or do they more closely resemble a the landscape-disfiguring tower of a power station? It rather depends upon where you choose to stand.

The entry point itself feels like a kind of cut, a species of unwarrantable violation. It is as if the steel walls might have been prised open at that very moment, by the mighty fists of Serra, as if to allow you – and none but you – to enter. Then you do enter. And you walk around a bit. You walk round and round, gaining a bit of speed, thinking of crazed bikers circling and ever-rising up some Wall of Death. The walls seem to be leaning in on you – you might be under observation. They list and lean, sometimes backwards, and then forwards, drunkenly. You feel enveloped, wholly psychologically enclosed. You begin to feel slightly vertiginous, slightly undermined by the scale and sheer soar of the thing.

Yes, it is strange how you feel when you are inside one of these pieces: not only slightly disembodied and set apart from the world, but also slightly beyond the grounding normality of clock time. It's as if, as you walk and walk, you have happened upon a new way of measuring duration.

One of the most disturbing aspects of Serra's work is that there is no welding to be seen. The two giant walls of steel that make up these two torqued toruses are tagged together, not joined. They are quite thin, too – no more than a couple of inches thick. What it means is that, essentially, these walls are doing nothing but holding their own, bearing down upon their own mighty, imponderable weights, and that helps to induce an additional modicum of fear.

That's the first two pieces. The third giant piece, Open Ended, is in the second gallery, and is even longer and more intricate than the other two. With the first two, you entered a central chamber, and saw all that there was to be seen at once. There was nowhere else to go other than the space you could see before your eyes. The walls may have been troublingly illusionistic – you may not have quite known whether they were facing this way or that; you may have been troubled by the fact that the walls, when viewed from outside, looked bizarrely different from those same walls when viewed from inside – but the space itself, once you'd entered it through that surprising cut at one end, was readable enough at first glance.

Open Ended is quite different. It's long, like a great, black beached whale, brutally felled, long and high-curving, lying on its side like something monstrous that might, or might not, just be sleeping. When you enter, you proceed between two high, curving walls that make you feel as if you, too, need to bend sideways in a strong wind as you walk. Then, after pacing steadfastly for what seems like a considerable distance, you come to an abrupt stop. The maze has suddenly turned back on itself, so you walk back in that same direction, except that this time the walls lean differently, and the width of the tunnel-like space along which you are walking has now changed to something narrower and more claustrophobic.

Having done that much, you are abruptly stopped again by a narrow wall of steel that causes you to turn back on yourself for a second time, and now the space you tread is almond-like in shape and... Where exactly will all this end up? Yes, you do wonder that to yourself. Eventually, you reach an exit point, and all of a sudden you are disgorged into the bright, relaxed space of the gallery, with all its hubbub.

Now, here is what is so interesting about this piece, and I give you this information in order that you can surpass me when you walk through it. I was so fascinated and psychologically bewildered by its extraordinary doublings-back upon itself that when I went through for a second time – I would suggest you do it at least three times – I decided to do a quick drawing of the space as I moved through it, in order to whittle it down to human size. I tried – and then tried again. I never quite managed it. I was simply not capable of sketching it in a way that enabled me to visualise exactly – including every single way in which those walls had changed direction – what I had been through.

I stepped out into the open air, defeated, outwitted, psychologically disembowelled. I gingerly touched my forehead before I put on the cycle helmet. Yes, just as I feared, I had the Mark of Serra branded into my brow.

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Richard Serra, Gagosian Gallery, London WC1 (020-7841 9960), to 20 December