Country diary: wild swimming in the eye of the beast

It’s early morning, and I’m floating on my back, a speck in a dark eye of water, surrounded by the green, foliose skin of a vast beast. If I turn my head to one side I can see a wooded moraine, the lower rim of the eye. To the other side is a jutting brow of pale rock rising above the tightly packed heads of oak, ash, beech.

This is Gormire, an oval glacial lake lying beneath a 140-metre cliff – the rim of the North York Moors plateau, in which the entire 55m-year span of the Jurassic is stacked in layered gritstone, limestone and muddy shale. It is undoubtedly one of the best swimming spots in the country.

From the precipitous viewpoints above, the deep water sometimes glows green, sometimes flashes blue, but usually it is dark. It has no apparent tributaries or outflows, but somehow brims clear, year-round, from an invisible source.

Related: North York Moors national park: a ranger's guide

Unsurprisingly, stories flow from this apparent mystery. They say the lake is bottomless. They say it is a portal to hell, used by the devil himself. They say that a goose that once disappeared beneath the surface emerged, minus its feathers, in a well 12 miles away.

They say the pike here are big enough to take a small dog. They say the water teems with leeches; and there’s truth there at least – I’ve seen some the length of my hand, blobbing and ribboning with gruesome grace over the sand, though they’ve never touched me.

A few weeks ago, before the solstice, the steep path down the cliff rang with birdsong, but now the air is hot and heavy with imminent change, and for a moment everything is so breath-held quiet that hanging there, in the middle of the eye, in the middle of this strangest of years, I can almost believe that if the beast is going to blink, it might be now.

It turns out that something has changed, but I only see it after I’ve dried and dressed: new official notices, which I’d failed to see on my walk down. They say the lake is cold and deep, and there are no lifeguards. They say it’s not safe for swimming. They say stay out of the water.