Edmunds first described Bosporthennis Quoit in 1848 as a small dismounted quoit. In 1872, WC Borlase excavated the site and recorded finding an urn and sherds of pottery along with calcined bones within the chamber. He also recounted a story of a miller who attempted to trim the corner of the capstone to make a millstone but later abandoned the idea. Daniel, writing in 1950, suggested the site should be classed more accurately as a large surface cist rather than a true quoit. The chamber itself measures about 1.2 metres high, 0.9 metres wide and 1.5 metres long, and the nearly circular capstone is 1.5 metres in diameter and about 0.15 metres thick. The structure has several notable features. The slab on the south-west side measures 1.8 metres long, while the supporting stone opposite is missing, leaving the cover stone lying prostrate on that side. The north-west end is formed by a single upright stone, and the south-east end by two stones. Around these remains lies a heap of earth and stones, which appear to be the remnants of a barrow that once covered the chamber. The Ordnance Survey mapped the site in 1961 and confirmed most of Edmonds’ description, although they noted that the single side stone was on the south side rather than the south-west. The large western stone and the side stone are upright and rest against each other, while several smaller stones at the eastern end are almost certainly no longer in their original positions. In 1971, Russell listed the remains of the cist and the displaced capstone, and the Ordnance Survey revisited the site in 1975 and recorded no significant changes since their earlier survey. Weatherhill in 1977 suggested that Bosporthennis represents a transitional monument, too small to be a Penwith chamber tomb but too large to be considered a simple cist. The CCRA planned the site at a scale of 1 to 50 in 1983, noting the south-east end stone appeared displaced and that clearance stones had been dumped on the capstone. They observed that the cist lies slightly south of the centre of the mound. Bosporthennis Quoit stands in a small rectangular field enclosed by massive, high walls. The orthostats are still deeply embedded in an ovoid barrow of stone and earth measuring 6.7 by 5.8 metres and reaching a height of 0.8 metres, with the scant remains of a kerb visible. In 1961, the Ordnance Survey recorded a larger barrow mound extending south from the chamber but never higher than 0.1 metre, nearly ploughed out over time. Later surveys confirmed this observation, and further planning in 1983 measured the mound overall as 6.7 by 6.1 metres. If the kerb continued in a complete circle, the mound would have been approximately 5.5 metres in diameter. Pronounced B’zprennis, the monument is now thought to be the remains of a Bronze Age entrance grave. The capstone was trimmed into a circular shape, likely intended for use as a millstone but never removed from the site. Unlike many other quoits in Cornwall, Bosporthennis retains much of its surrounding mound. Four uprights of the chamber remain along with the fallen capstone. Nearby, at the eastern foot of Hannibal’s Carn, lies a scattered settlement of at least three Late Iron Age courtyard houses, including one with walls up to 1.5 metres high and another containing a medieval cowhouse. Within the settlement is the Bosporthennis Beehive Hut, sometimes identified as an above-ground fogou, though this interpretation is considered doubtful. Excavations in 2009 confirmed that Bosporthennis Quoit was likely oval in plan with a possible northern entrance, later blocked in the early medieval period, suggesting it may have been either a large cist or a small entrance grave comparable to examples at Bosiliack and Brane.