Nab Tower: Originally intended to form part of Britain’s war time
submarine protection,to prevent U-boats running in undetected
on the approaches to the south coast, it was decided to construct
watchtowers and string metal cables between them. Building
work started in 1918, which the historically astute will recognise
as the year in which the Germans threw in the towel.
A couple of towers were finished, but only one ever made it to
the water. The rest were eventually dismantled, a shame
considering that 21 years later the U-boat threat returned.

The remaining floater was towed out to Nab Reef and sunk close
to Nab Rock, with a light attached to warn ships of the obstruction
close by. The light was manned by three men, joined for a short
time during World War Two by a gun crew which shot down three
and a half (the other half was assigned to a boat crew) enemy aircraft.
There is still a light on top, operated by Trinity House, and it
looks like a shiny new bonnet on an abandoned car.
The Nab Tower has been at sea for 84 years, been shot at by aircraft
and boats and even struck by a freighter which failed to see
the light in 1998, so you have to feel a little sorry for it. Below the waterline, the tower sits
on a hollow circular pedestal of concrete that steps down to the sea floor at 24m. Because
of its location east of the Isle of Wight, tidal currents make diving tricky. It is best done on
slack water during a neap tide; the smaller the tidal range, the better. Slack during a spring
is virtually non-existent, and you wouldn't want to be swept off the structure, as it could be a
while before anyone found you. The first ledge is in 6-8m, depending on tide, the next at
11-13m. The last is in around 16m and then the sea floor is reached at 24-27m.