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A map of Horsea Island, 1945; detail omitted for security reasons.
Sea Survival Centre slipway, with Paulsgrove landfill site reclamation beyond
Horsea Island was an island located off the northern shore of Portsmouth Harbourmarker; gradually subsumed by reclamation it is now connected to the mainland. Horsea falls within the city of Portsmouthmarker and is wholly owned by the Ministry of Defence as part of the HMS Excellentmarker shore establishment, which maintains its headquarters on Whale Islandmarker.


Horsea was originally two islands, Great and Little Horsea, the former large enough to support a dairy farm. The islands were joined to form a torpedo testing lake in 1889, using chalk excavated from Portsdown Hillmarker, 1 km to the north, by convict labour. A narrow-gauge railway was constructed on the site by the army to distribute the chalk. Although the lake length was increased from to over in 1905, rapid advances in torpedo design and range had made it all but obsolete by World War I.

In 1909, the island became the site of one of the Navy's three high-power shore wireless stations, which saw it populated with dozens of tall masts. In the 1950s the lake was used in the testing of improved Martin Baker Ejection Seats, following catapult launch mishaps on carriers in which Fleet Air Arm aircrew often sustained serious compression injuries to the spine after ejecting from submerged aircraft.

After closure of the telegraphy station in the 1960s, the northern part of the island became home to HMS Phoenix, the naval school of firefighting and damage control. The school comprised a number of steel structures (:'trainers'), simulating three decks within a warship. Fires were set in the trainers for the purposes of instruction in various types of firefighting.. The kerosene and water mix burned in the trainers, known as sullage caused significant water and air pollution and created a health hazard for the staff exposed to the fumes for protracted periods. In 1994 the school removed to a modern gas-fired trainer on Whale Island as part of a consolidation and cost effectiveness initiative. The new facility is known as the Phoenix school of Nuclear, Biological and Chemical Defence, damage control and fire fighting. Responsibility for training and site management was contracted out to Flagship Training UK, which was taken over by Vosper Thorneycroft in September 2008.

Current use

The original island site continues to be used by the MoD, with a number of facilities on the site predominantly focussing on diving and underwater engineering. Infrastructure includes training facilities as well as workshops, decompression chambers and equipment testing capabilities. Organisations on the site include:
  • The Superintendent of Diving, a Commander, Royal Navy, who is responsible for safety and standards of diving in the Royal Navy and Royal Engineers.
  • Maritime Warfare School delivers the Defence Diving School, providing new entry diving training for RN and RE divers as well as promotion courses as divers progress in their careers.
  • Headquarters of the Fleet Diving Squadron, which delivers diving, underwater engineering and bomb disposal capabilities in the UK and overseas using the Northern and Southern Diving Groups, and the Fleet Diving Group.
  • Southern Diving Group (East)
  • Fleet Diving Group
  • The Sea Survival section of Phoenix.
The lake is also used by civilian diving schools and clubs and is open to recreational divers on a regular basis.


In the early 1970s, the tidal mudflats between the island and mainland at Paulsgrovemarker to the north were reclaimed, much of the area destined to become a landfill site, the remainder to form the Port Solentmarker leisure complex. The landfill site closed in 2006, and waste is now incinerated at a plant in east Portsmouth. The northern part of the reclamation has been developed as 'Port Solentmarker', a complex comprising a marina, multiplex cinema, housing, retail outlets, and some business units known collectively as the 'North Harbour Business Park'. The rest of the landfill site is being developed as a recreational park featuring woodlands and meadows.


The solid geology of the site is Upper Chalk, covered by post-glacial drift deposits comprising mostly brickearth, a loess from the west of England eroded and deposited downstream by the river system which once occupied the area now known as the Solentmarker and its margins before inundation by the sea. Much of these deposits were covered by the chalk fill imported from Ports Down to create the torpedo lake. Where still exposed, beyond the lake at the eastern end of the site, the brickearth comprises fairly equal proportions of sand, silt, and clay, with occasional flints. Owing to the high clay content, the rock is of low permeability, occasioning flooding after prolonged rainfall during winter. In the extreme south east corner of the site, brick and concrete rubble has been used to construct the bunding to enhance protection from the rising sea.

Horsea Lake

The lake, which contains a wealth of marine life, has a number of items placed for diver training including a helicopter, vehicles and a 200 year-old ship wreck, placed in the central section. The level of the lake is maintained naturally by two submerged freshwater springs.


Horsea's SSSI area
All of the undeveloped area to the south of the lake, with the exception of the helipad, forms one of the few terrestrial parts of the Portsmouth Harbour Site of Special Scientific Interest (SSSI) on account of its calcicole flora and fauna which have flourished on the imported chalk. Notable fauna include the Small Heath butterfly Coenonympha pamphilus recently designated a UK BAP Priority Species by DEFRAmarker on account of its increasing scarcity (it is now extinct on Ports Downmarker), and the micromoth Eulamprotes immaculatella, Horsea being the latter's only known habitat in Hampshire. The SSSI area is in poor condition however, owing to over-grazing by rabbits in the smaller meadows, and under-grazing in the largest meadow east of the helipad, although the latter problem is has now been addressed with the introduction of four unshod horses in 2008. Six sheep and a number of small ponies were introduced to the smaller meadows in the summer of 2009.

The south-eastern extremity of the site, beyond the SSSI, has been used as a trials site by Butterfly Conservation for the evaluation of new disease-resistant elm cultivars since 2001, and currently accommodates 30 trees comprising 14 cultivars and exotic species. The site's English Elms Ulmus procera, were home to a colony of rare White-letter Hairstreak Satyrium w-album butterflies until 2000, when a resurgence of Dutch elm disease killed the remaining semi-mature trees. The butterfly survives at IBM North Harbour, and may recolonize Horsea in years to come.

Portsmouth Football Club

Proposed stadium site looking north
In October 2007, Portsmouth F.C. announced plans to build a 36,000 seat football stadium on the south-east corner of Horsea Island, near to the M275 motorway. The proposed new stadium was planned for completion in 2011 , and a formal planning application was submitted to Portsmouth City Council in January, 2009. However, owing to the economic recession, the plan was postponed indefinitely only two months later in favour of expansion of the existing ground, Fratton Parkmarker, to a capacity of 30,000.

The Horsea stadium proposal is beset by serious logistical problems, notably the paucity of access by road and rail (the nearest railway station is at Coshammarker, 6 km away), but indications are that a junction would be constructed on the M275 to access to the new stadium and also facilitate development of the nearby Stamshawmarker area. A railway station at Wymeringmarker has also been proposed, but this would still be 3 km away from the proposed stadium. It was originally planned that the stadium would be funded by housing development on the island, but this proposal was dropped in favour of retail development, itself a matter contention because of the detrimental effect it would have on existing shopping facilities. Nevertheless, most fundamental problem remains the site's vulnerability to flooding. Less than 1 m above sea level, part of the site is already regularly inundated by sea water on spring tides. Moreover, in a study led by the US National Center for Atmospheric Researchmarker, sea levels are predicted to rise six metres by 2100 which, if correct, would see Horsea, like the Maldive Islandsmarker, submerged before 2040 [212716].


  1. The Portsmouth Papers, No. 36: Horsea Island and The Royal Navy. Portsmouth Museums, Portsmouth.
  2. Hooper, S., Smith, D. & Tomlinson, N. (1991). The COPSE* Report (*City Of Portsmouth Survey of the Environment). Portsmouth Urban Wildlife Group, Portsmouth. (Private publication)
  3. Overpeck, J. T., Otto-Bliesner, B. L.,Miller, G. H., Muhs, D. R., Alley, R. B. & Kiehl, J. T. (2006). Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise. Science, 24 March 2006, Vol 311, no. 5768, pp 1744-1750

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