A strong earthquake measuring 5.7 on the Richter scale shook the Libyan Sea and the southern Greek island of Crete early on Friday, but there were no immediate reports of damage or injuries, the Athens Observatory said.
The quake had an epicentre 400 kilometres (249 miles) south of Athens, near the island of Gaidouronisi, and struck at 2:17 am (0017 GMT).
"Earthquakes of similar size have been recorded in this area in the past but not recently," observatory seismologist Yiannis Papis told AFP.
"Though this was a strong quake, it is positive that it occurred undersea and not near the coast," he said.
The quake was mainly felt on eastern Crete in the areas of Iraklio and Lasithi, Papis added.
Greece sustains more seismic activity than any other European country, accounting for half of the continent's recorded earthquakes.