As a relative newbe I'm not exactly sure if this has ever been discussed before (could not find anything about it in the search option) but I discovered something on Dutch and German Wikipedia (not the English one mind you!) that may interest you.
I was fooling around a bit and typed Plakias in the various Wikipedia's and some interesting information popped up.
You all know the two openings in the mountain at the end of the Plakias bay. Sitting at Ostraco's terrace one day, sipping our cold beers (sigh!) we noticed that in the smaller opening on the right there seemed to be a rectangular shape (see picture) and the misses and me fantasized for ages what that shape could be. (btw, what a great way to pass a lazy Plakias afternoon) We once took the boat to Prevelli and had a chance to take a better look at it and were convinced that this structure was man made but it stopped at that, no more information.
Wikipedia says that once there was a coal mine run by a German businessman between Plakias and Lefkogia and that there was a lorry track through the mountain to allow ships to take in the coal, they had to do it this way because the water near the coast is to shallow. Apparantly the coal was of such poor quality that it could only be used in steamships. This coal business stopped in 1930. The mine was closed of with concrete only at the end of the 80's because a child drowned there. During world war 2 the Germans made the tunnel longer and made a second and bigger opening in the mountain to enable German submarines to take in food, water and torpedos.
The German Wikipedia also states that the name Plakias comes from a certain kind of sand that is found in many places in the bay. In German it says: 'Sandstein-Platten' According to the dictionary this means: 'Sandstone-tiling"
Between brackets they also call it: 'Naturzement' , in English this should be: 'Natural cement'.