This involves placing a huge dam across the Strait of Gibraltar, and lowering of the surface of the Mediterranean Sea by evaporation.
The geometry of the Mediterranean Basin makes this project a possibility: the sole entrance channel (apart from the Suez Canal) is only about 320 metres deep and 13 kilometres wide.
The mediterranean is large and relatively shallow; a lot of land would be created without too much effort - and the land would be in a prime location.
A similar project could join Sweden with Denmark with a dam. This span is already bridged. Other possible projects include dams as Yemen and Dubai - damming up the Red Sea and the Persian Gulf.
In the case of the Strait of Gibraltar, it has been estimated that normal evaporation would result in about 0.5 metres per year of sea level fall - at that rate, it would take many hundreds of years for the basin to empty.
Evaporation rates could be increased by using black evaporation rafts - that float just below the surface.
Perhaps eventually, giant evaporation pools could be created inside - to increase the rate of evaporation. This would eventually allow more hydroelectric power to be generated by allowing sea water in at a greater rate.
If substantial sea level lowerings are to take place, these become projects that must not go wrong - and thus need a large safety margin. For example, a major influx of the Atlantic Ocean into an enlarged Mediterranean Basin could be a major catastrophe.
The issue of safety margins is often complicated by the presence of boundaries between two tectonic plates. Africa is approaching Spain at a rate of about 4mm per year. This might result in earthquakes in the vicinity of the proposed dam. A similar issue affects both the Red Sea and the Persian Gulf projects.
It also needs to be firmly established whether such basins leak anywhere - and if so, what can be done about that. Freshwater springs are known to exist in the Mediterranean sea: in 2003 France harnessed a submarine freshwater spring gushing 100 litres/sec at a depth of 36 metres below the surface. However whether there is any other underground oceanic influx is not known with certainty.
A large-scale ocean reclamation project would probably result in a large increase in salinity of the enclosed sea - making it rather like the dead sea.
An alternative to ocean reclamation would be tidal hydroelectric dams in those areas. However, if hydroelectric power is the only goal, it would probably make more sense to dam rivers e.g. the proposed Severn Barrage. Generating tidal power does not require a large enclosed area.
As with most other large ocean reclamation projects, these ventures would most likely result in an increase in global temperature - and thus would help to melt the ice caps and thus end the ice age.
They would also contribute slightly to global sea level rise - due to the water they displace.
The Hormuz Strait Dam Macroproject
Red Sea Megadam Would Be Irresponsible - criticism