It's an annual spectacular underwater show that occurs in the shallow waters off the beaches of Port Phillip Bay.

Great Spider Crabs move en-masse across the bay's sandy bottom, walking, feeding, clambering over one another with long legs and forming a living pyramid of up to 10 crabs deep. And it's happening now in the waters off Blairgowrie.

The event, which occurs around May and June, has attracted the interest of the naturalist David Attenborough and the BBC, but for a large number of Melburnians it's an underwater secret.

Underwater video taken just days ago by diving enthusiast and aquatic scientist Sheree Marris reveals the mass gathering.