While I was in Greece at the begining of April for a while a friend of mine and we visited on my boat an island in the Aegean called Giaros (ΓΥΑΡΟΣ).
The island is located about 30 miles NNE of Cape Sounion.
This island was dedicated from 1947 to 1970s to be a prison to political prisoners in Greece. The prison and the buildings were designed by British architects and the building although it was done by the prisoners it was supervised by British civil engineers.
The island now is abandoned.
here are two small videos ... hope that gives you an idea.