This week I have visited the sea serpent in his home. I have dallied with monsters from the deep. I have been, in sober truth, very close to giants. I have been surrounded by five of them in a feeding frenzy, and I was home in time for tea. It was a perfectly amazing afternoon, particularly as these 15-footers were dismissed as “tadpoles” by my guide, who meets 35ft giants regularly.

There is a residual belief, something that lies deep in the human mind, that anything, absolutely anything, can emerge from the ocean and get us. The sea serpent, the Kraken, Grendel’s mother and Jaws: it’s all the same story. And yet, when we are away from the water, our civilised 21st-century minds are unable to cope with the idea that there are seriously big animals just off the shore of our own country, that a two-hour bounce in a boat from Penzance can bring you face to face with something unimaginably huge.

A fin, dark and triangular, breaking the surface. A tail, ditto. Measure the distance between them by eye and you have the length. Then stare off the boat and wonder that such creatures should exist within sight of places that do cream teas.

These were basking sharks – the planet’s second largest fish after the whale shark – not creatures that get blown here by accident, but monsters that have every right to ply their trade off the West Coast of Britain. And there, a third triangle, as the pointed snout broke the surface. That’s how the sea-serpent myth began, it seems: two or three of these creatures moving in line astern give the impression of a single giant.

It was enough to get The Sun in a tizz the other week, when a British basking shark was resoundingly identified as a great white. Quite right to get excited in a way, for these are truly disconcerting things. But they don’t eat human beings. They filter plankton, cruising along just below the surface and hoovering up the grub in an enormous gape. So here’s a would’ya-believe-it fact: a basking shark can filter the equivalent volume of an Olympic-size swimming pool in an hour.

I was actually looking for dolphins, having been tipped off by my friends at the Sea Watch Foundation, a marine conservation charity; and I was travelling with the delightful Marine Discovery Penzance. But when you look for wildlife, you revel in what you find, not what you want to find, and the sight of this quintet of immense beings was as powerful a revelation as you could wish for.

It’s one of the things I have noticed as I travel about seeking wild things: you need only take a tiny step, you need only to cross the boundary, to alter your entire perspective on the world. Duck your head below a reedbed; look down over an expanse of rainforest; take to the air on a microlight.

And here, in this boat. Now I am not much of a one for boats, to tell the truth. I enter them on sufferance occasionally and in single-minded pursuit of wildness. But here, with the shore still in plain view, I found myself in a a genuinely different world: gannets raining down into the sea with their spear-point beaks, shearwaters shearing the water all around, and beneath us, still more unseen monsters. You don’t need to go into space to seek alien beings and unfathomable ways of coping with the Universe; you just need to cross one of those boundaries. I bounced back teawards with a mind full of wonder.

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