Quoth the crayfish, "Nevermore"

There are about eight native crayfish species in Russia, and those crayfish have made an impression on the vernacular. This expression means, "Never":

когда рак на горе свистнет (kogdá rak na goré svístnet) — “When the crawfish whistles on the mountain”.

For more ways of saying "Never," see here. It misses a favourite of mine, though. Apparently, in ancient Rome, an expression for "never" was, "When a mule foals."

External link

When Pigs Fly, Crayfish Whistle, and It Snows Red Snowflakes