Underneath the A310 in Twickenham, in the grounds of Radnor House prep school, lies the grotto of Alexander Pope. It once looked out over the Thames, but now its view takes in the walls of the sixth-form art block and an astroturf sports pitch. But the magic of what Pope called his ‘shadowy cave’ is not lost.

The grotto smells of flint. Its walls are encrusted with geological curiosities. There is a piece of basalt hacked from the Giant’s Causeway and there was once a stalagmite from Wookey Hole, supposedly shot down from the roof of the cave at Pope’s request. It is dark, cold and dusty, and sorely in need of restoration. An iron gate usually keeps pupils out, but it was unlocked for my friends and me when we went along on a school open day a couple of months ago. Our arrival at reception had caused some confusion – too old to be prospective pupils, too young to be parents – we felt like interlopers. And I felt like an interloper in the grotto too – it is so much Pope’s personal fantasy.

Pope was 4'6", self-educated, hunchbacked. As a child he contracted Pott’s disease, or tuberculosis of the spine, from the milk of his nurse, Mary Beach. In later life he put up a tombstone for her in Saint Mary’s Church, five minutes’ walk downstream from the grotto. As a Catholic, Pope was unable to vote or go to university. He was born in the year that the Catholic James II was deposed. Throughout his life he was encumbered by his faith as much as his ailing body.

In the spring of 1719, at the age of 30, Pope moved to Twickenham, a fashionable retreat two hours from London by boat. He was at work on his translation of the Iliad, which gave him a sizeable income (by some estimates, more than £1 million in today’s money). He built himself a villa in the Palladian style, with a garden that sloped down to the Thames. He later leased the land at the back of the villa, so that he might have a more secluded garden, sheltered from the gaze of passing river traffic. The five-acre ‘wild’ garden (one of the first of its kind) was separated from the villa by a narrow lane (now the A310). To reach it without crossing the road, Pope built a tunnel from his cellar. ‘He extracted an ornament from an inconvenience,’ Samuel Johnson wrote, ‘and vanity produced a grotto where necessity enforced a passage.’ According to Swift, Pope turned ‘a blunder into a beauty which is a piece of Ars Poetica’.

It took five years to build. At first, Pope saw it as a musaeum or nymphaeum. By inviting the natural world into his cellars, he hoped the Muses might also pay a visit. The opening of the grotto was marked by a plaque with a quotation from Horace, ‘secretum iter et fallentis semita vitae’, which Joseph Spence translated as: ‘A hid Recess, where Life’s revolving Day,/In sweet Delusion gently steals away’. It might have been home to the Muses, but it was also a place of retreat.

Over time the grotto’s purpose changed. In 1739, Pope took the waters of Hotwells Spa in Bristol, and was transfixed by the geology of the Avon Gorge. After that, the grotto became a shrine to the majesty of geology. Pope was influenced by his friend William Borlase, an antiquarian, who espoused ‘physico-theological’ ideas about geology as evidence of the work of God. Pope decorated his grotto with crystals, shells, ores and spars, ordering shipments of material from distant parts of the country. After a spat with his friend Lady Mary Wortley Montagu, she described it as ‘a palace beneath the muddy road’, which was ‘Adorn’d within with Shells of small expense/Emblems of tinsel Rhyme and trifling sense’.

There is a drawing by William Kent of Pope at work in his grotto. He is writing with one hand and resting his head on the other. The ‘Aerian grot’ is lit by a lamp – presumably the alabaster lamp described by Pope in a letter to Edward Blount in 1725. At the top of the drawing’s ink washes, fantastical winged insects flit about. The grotto and fantasy are as one.

Pope died in 1744. His villa was demolished in 1807 by the then owner, Baroness Howe, who resented the stream of visitors who came to pay homage to the poet. But the grotto, improbably, remains. It is a testament to an 18th-century poet’s imagination, but in its cold recesses an older history – from a time before history – reveals itself. After forty minutes, we came out and stood on the astroturf in the sunshine to get warm.

You can donate to the Pope’s Grotto Preservation Trust here.