“Skandalopetra” is freediving with a stone-plate (“petra”) as a ballast, which in turn is attached with a rope to the boat.
This method was used by Greek sponge-divers to descend to the ocean floor to collect sponges.
A few pulls on the rope by the freediver indicated to the Kolizeri (rope-guy on the boat) to bring him back to the surface.
Modern-day Skandalopetra as a sport is done in a bathing-suit and nose clip (and no sponges are involved). No wetsuit, mask or fins are allowed.
The biggest challenge?
The ever-present thermocline in Greek waters, as it hits you like a brick wall, and makes equalization a serious issue.
No wonder all the traditional sponge-divers had permanently damaged eardrums.
Sponge diving was mainly practiced in Kalymnos and Symi (both near Rhodes in the Aegean Sea).