Scuba diver PT Hirschfield was swimming by the Blairgowrie Pier in Australia when she stumbled upon a horrifying, and unusual, sight: A giant pile of hundreds of spider crabs.

The crabs are known to gather into huge groups before their annual migration. Some believe that the animals find safety in numbers when they're shedding their shells; apparently the process doesn't take long, and the crabs disperse in under two hours. 

“Unfortunately, not much official research is done on the spider crabs, so it’s more guesswork based on observation than anything,” Hirschfield told GrindTV. “I hope to spend a solid week underwater with them when the migration proper happens to continue studying them more closely, but the pre-migration stages remain somewhat of a mystery.”