Hellanicus of Mytilene, the main town on Lesbos, was a historian of the late 5th century BC and is credited with writing the first history of Athens. His importance is probably reflected in the frequency with which his work is mentioned. Unfortunately, only about 200 fragments of his work have come down to us as described by Robert L. Fowler. Tantalisingly, one of these fragments was entitled Atlantias. It is quite probable that this was written up to a century before Plato’s account, which would suggest that his story might not be the original invention suggested by many. Timothy Ganz notes one line that is particularly noteworthy, “Poseidon mated with Celaeno, and their son Lycus was settled by his father in the Isles of the Blest and made immortal.”
The earliest suggestion of Hellanicus offering a possible pre-Platonic mention of Atlantis was voiced by J.V. Luce in his contribution to Ramage’s Atlantis :Fact or Fiction[522.72]. Andrew Collins dismisses this reference as irrelevant, whereas Rodney Castleden is inclined to identify some similarities with Plato’s tale and in addition suggests that an even earlier reference in a fragment from the Oxyrhynchus Papyri may have inspired Hellanicus. Oxyrhynchos was a mainly Greek-speaking city in ancient Egypt whose rubbish dump was the source of the papyri referred to. Peter Parsons has recently written a book about the city.
The final haul from that site amounted to 500,000 papyri fragments which filled 700 boxes. This treasure trove is now being studied and gradually published by the Ancient Lives Project(a) at Oxford University. Since 1898, when the first volume was produced, over seventy five volumes have been produced and are now arriving at a rate of more than one a year(b).
(a) https://www.ancientlives.org/ (offline April 2020 until later in year)