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.
The Swallow holes are a unique geological phenomenon where seawater is continuously falling in to the sink holes only to disappear. In 1835, an English resident, Mr Stevens, spotted the potential for water power at the point where the sea water disappeared, he built some huge sea mills to grind corn and named the mill Stevens Mill after himself. However, there is also a theory that a Angelos Trombetas built a mill on the same site, where his company also ground wheat and barley which he used to brew beer. The water would flow swiftly through the artificial channels and the wheel would start spinning. Another water mill, a little nearer to Argostoli, was added in 1859. This mill, with the water wheel inside the building, is now a popular café bar in the summer months. A derelict taverna sits on the site of the first mill, though the once-restored wheel is still present, next to the swallow holes.
Throughout the years many scientists have tried to explain the phenomenon. The famous geologist, Miliaresis, studied the effect of earthquakes on the phenomenon during the same period. He observed that even during large earthquakes, like the one in 1867, the mills continued to work unhindered and there was no change whatsoever to the flow of the current. Later on, during the terrible 1953 earthquake, again there was no change to the flow of the sea at the swallow holes or sea mills. The only thing that was destroyed was the water wheel, which was replaced by the one there today. Shortly before the 2nd World War, the mills were actually in use and served also as an ice factory. Many attempts have been made to explain the phenomenon. Ansted suggested the cave theory in 1835, later in 1842 the geologist Davy suggested that the phenomenon was linked to earthquakes, and Strickland even proposed the plutonium theory but, no one came close to explaining the phenomena.
However, in 1963, Austrian geomorphologists added a purple dye in to the water and followed its course only to make some surprising discoveries. It traveled in underground rivers then mixed with rainwater and finally reached the Melissani Lake almost fifteen kilometers away in its semi-stitleed form. From there the water gushes out at the village of Karavomilos near Sami, forming a nearly completely circular fresh-water lake, which empties into the sea in Sami Bay. All in all it had taken the purple water two weeks to make this trip across the island.
The famous Katavothres or swallow-holes are an extremely rare geological phenomenon and is believed to be the only one of its kind.