“On the very scenic Fanari coastal road is the famous Katavothres (swallow holes) an extremely rare geological phenomenon . . .It is not known in any other place on Earth!
The seawater flows inland here to enter sinkholes below sea level never to return again, their depth and magnitude unknown.
On 26th February 1963 Austrian hydro geologists dropped 140kg of uranine, a very intensive green colour, into the Katavothres. The colour arrived at Melissani subterranean lake and the springs at Karavomilos fourteen days later, on 12th March 1963. This experiment proved that the opposite sides of the island were connected by an underground system.

more info here:

http://www.jr-creative-images.co.uk/Kefalonia%20Section/Map%20pages/Place%20info%20pages/Katavothres.html

Swallow holes

The island is known for a number of unusual geological phenomena, which have remained unexplained throughout the centuries. During this time, the underground waters have eroded the limestone rock formations, resulting in passages and cavities on the earth’s surface, as well as underground. Due to many earthquakes, most of the island’s caves are bottomless.

The phenomenon of Katavothres in Argostoli was known for many centuries and became a subject for scientific study. Going against the laws of nature, the sea water flows into small deep holes, known as the Swallow holes of Katavothres, in the entrance of Argostoli’s natural port.
— from http://www.kefapps.com/seawater.html

Cave system