The Royal Commission on the Defences of the United Kingdom had initially suggested the building of a barrier on the sands between Horse Sands Fort and Langstone Harbour. This became known as The Spithead Breakwater. Nothing was done in 1861 but the idea was resurrected in 1905 in the Owen Report of the Committee on Armament of Home Ports. The concern now was a real one, a mass (swarm) attack by hostile Motor Torpedo Boats who could inflict damage on the Fleet at anchorage in Plymouth Sound.
The plan was devised to sink a chain of 10 foot cube concrete blocks, each weighing 60 tons. These blocks were to cover the water between Southsea Castle and Horse Sand Fort, over 3,000 yards. There was then a gap, which would be covered by a boom defence, before another stretch of the barrier ran from No Man Land Fort to a location just East of Puckpool Battery on the Isle of Wight. There were some opening in the chain, but these were only 100 feet wide and by 1909 local sailors complained these access points were too narrow. The block channel shipping traffic through specific choke points which could be covered by coastal defences.
The concrete blocks were constructed on the shore to the West of Fort Gilkicker, close to the Submarine Mining Station, under the supervision of the Royal Engineers.
Recently the Solent Barrier has been often referred to as a submarine barrier. While it would later prove a valuable barrier to submarines, this was not the threat in 1908 when submarines were still in their infancy.
The East side of the barrier largely survives as it has been deemed as too expensive to demolish.
For more images of the original block see the short article from the Friends of Stokes Bay at https://www.friendsofstokesbay.co.uk/spithead-breakwater/.
The video recommended gives a good overview of this linear structure as it now survives. See https://www.youtube.com/watch?v=Hw_MDaxAkPI&t=27s