From Kielder Observatory, I can see Mars, Venus and Mercury, and all with the naked eye. I can’t see the rings of Saturn — but only because somebody has stolen them. The planets dangle in a line beside the forest track to the observatory, suspended from wooden gallows and wobbling slightly in the wind.

Anywhere else, a row of giant extraterrestrial gobstoppers would be a pretty surreal ornament on top of a remote fell. But Kielder isn’t anywhere else. It’s a whole world of strangeness, and after a couple of days here, the feeling that I’ve wandered into some kind of bizarre sci-fi universe is becoming ever more gripping. Take the observatory itself, which juts out from the heather behind me. Opened last year to architectural acclaim, it comprises several stripped-pine cubes and looks as if it arrived flatpacked from Ikea.

Half a mile along the track, I come upon another surprising structure, hollowed into the side of a crag. Through a tunnel I go, and into an underground dungeon, cone-shaped and weirdly windowless, with a hole at the apex through which the heavens seem to bulge, a ball of blue.

This is Skyspace, an installation by the Californian artist James Turrell, and when I stand in the dead centre, my head becomes an echo chamber, and every sniff or splutter booms like a thunderclap between my ears.

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Kielder is not what I expected, let’s put it that way. On paper, this looked the gentlest of jaunts — out into northernmost Northumberland to sample a new trail that circles England’s remotest body of water. Cloaked in our densest forest and darkest skies, Kielder Water may be man-made, but it’s as profoundly “in the country” as you can go. I packed my fattest novel and my thickest cagoule, and prepared to soak myself in superlative countryside.

Yet within 48 hours, I’ve encountered all manner of mad stuff, including a disembodied head as big as a house, a sofa in the middle of a sheep field, and a cloud of sinister butterflies, casting orbs of light through the woods.

From the minute I arrive, something feels odd. I drive the last 30 miles from Bellingham, through swerve after swerve of sheepy nowhere-land, jump out of my car at Hawkhope and drink in the lake. It’s a real catch-your-breath moment: six straight miles of royal-blue water, backed by an endless frieze of conifers. The breeze is so frisky, it blows my glasses away.

This is Canada-class scenery, but the longer I gaze, the stranger the view seems. To my right is a long, pink curl of concrete, Kielder Dam. And I begin to notice a subtle orderliness to the forest, the suggestion of straight edges where there should be bushy froth. The scenery is magnificent, but it is also a manipulation. The lake is a fake; every tree has been planted.

I strike out along the new Lakeside Way, a thick stripe of stone and cinders, designed for bikes, boots, even mobility scooters, that loops right around the reservoir — 27 miles in all. This southern shore has been wooing weekenders since Kielder Water was built nearly 30 years ago, and offers plenty of outdoorsy stuff. You can kayak at Hawkhirst Activity Centre, go mountain-biking on Deadwater Fell, feed the owls at the Bird of Prey Centre and defy death on the Calvert Trust’s treetop adventure course.

But the weirdness of the woods is enough for me. Out on Bull Crag Peninsula, I find myself in a silent grove of spruces, their trunks unnaturally straight and lofty. Some of the trees are daubed with peculiar runic hieroglyphs, others with vivid green numerals. Some are just stumps, scythed down in an apparently random act of butchery. It’s compelling — but, this time, it’s not an art installation. This is the everyday business of commercial forestry, and it’s contorted the landscape into a very unsettling place indeed.

Outside Kielder Observatory, I meet Peter Sharpe, chief curator for the forest park, who explains how the outdoor art programme here has become the most eyecatchingly ambitious in the country. With 250 square miles of forest to play with, why stop at sculpture? Projects such as the Observatory and Skyspace are daring pieces of contemporary architecture.

“Kielder is a challenging place,” Sharpe says, “because it’s designed to appear as natural as possible, for visitors and for wildlife. Yet the landscape is profoundly artificial. I think it’s why the art works so brilliantly here. After a while, you begin to notice geometric shapes in the forest, the way the trees have been lined up for felling. Almost all English countryside is man-made, but Kielder is a magnification of that. It plays tricks with your mind.”