Koufonissia comprise two isles, Ano (Epano or Apano) and Kato Koufonissi (5.7sq. km. and 4.4sq.km. respectively), which are separated by a narrow strait 200m wide and the island of Keros.
They are both part of the Natura 2000 Network of natural habitats and feature a varied coastline of soft limestone that has been eroded by the sea and the rain, making impressive geological formations.
The landscape is smooth and stony, with low vegetation that becomes thicker in the eastern part of Ano Koufonissi.
According to one view the name is due to the cavities and caves along the coast (koufio=hollow) that appeared huge to pirates that roamed the area, making them believe that the islands were hollow. Presently, these geological formations, sometimes playful and sometimes forbidding, are impressive natural sights interjected among enchanting beaches.