Rockall is a very small island lying approximately 300 km (186 miles) west of St Kilda. St Kilda is 66 km (41 miles) west of Benbecula in the Outer Hebrides, and 430km (267 miles) from the nearest point on the Irish mainland. The outcrop measures circa 25m (82ft) on its north-south axis and 22m (72ft) on its east-west axis; the summit was confirmed at 17.15m above mean sea level by data collected during this expedition, having been officially 19.2m (63ft) ASL prior to the summit being removed in 1971. It is situated on the Rockall Bank, an Atlantic ridge separated from the European continental shelf by the Rockall Trough. The island is the core of an eroded volcano that erupted around 55 million years ago. The location was confirmed in 1967 as at 57°35′50″N, 13°41′13″W (Ordnance Survey, British Grid MD 96390 16624).
The occupiable area of Rockall, named in 1955 as Hall’s Ledge after the first recorded person to land there, is just 3.5 metres by 1.3 metres (11 foot by 4 foot), and is 4 metres (13 foot) below the summit. There are no trees or bushes on the rock - just algae, seaweeds and one black lichen. Just over 20 species of sea bird and only 6 species of animal have been recorded on or near Rockall. Greenpeace placed a solar powered beacon over the frame of the exisiting navigation aid in 1997, and returned to upgrade this light in 1998. This was the only permanent mark of human occupation on Rockall until it too succumbed to the ravages of an Atlantic storm two years later.