The once important sanctuary of Poseidon Hippios on Cape Tainaron is accessible via the current village of Porto Kayio (the ancient Psamathous). From there it is another 15 minutes to Porto Sternes, with the actual sanctuary for Poseidon. The history and the original shape of the sanctuary is somewhat controversial: if you come to the triple bay of Port Sternes from the mountains or from the sea, you will see the remains of an early Christian church at what is perhaps the most striking place, complete with apse at the end and an arched entrance. Closer examination reveals that a great deal of antique building material has been incorporated into the church and that in any case the entire north wall gives the impression that it is antique, rather than built with antique remains. Possibly therefore the entire north wall of the church is still a remnant of the ancient temple. It would then be after the collapse of the temple that a church was built on top of the ancient remains.
Still, the above scenario is considered unlikely today. Almost 50 meters from the church is the cave through which Herakles is said to have brought the hellhound Kerberos to earth. The preserved traces in front of the cave (especially the clearly leveled ground) suggest the former existence of a building, probably the actual temple of Poseidon. Only this complex of temple + cave can be what Pausanias has called a “temple in the shape of a cave” (3.25.4).
The worship of Poseidon Hippios is completely different from that of the normal sea god Poseidon. The god was originally worshiped as the god of the underworld, as was Poseidon Hippios in Arcadia, who was married to Demeter and had by her a daughter named Despoina. For that reason, the cave at the sanctuary was also one of the gateways to Hades, even though Pausanias scoffs that there is no road down through the cave. It is likely (as suggested in some ancient sources) that in this shrine people could summon the dead and communicate with them.
Around the sanctuary you can still see many traces (especially leveled floors) of the ancient settlement of Tainaron, which later moved to Kainepolis. A mosaic floor from the Roman period can be seen at a distance of 100-200 meters from Porto Sternes.