I was at the beach one afternoon in Australia Adelaide, I was young so I didin't think much of it but when I was running towards the shore (About average belly button deep) the sand under my feet was collapsing, I don't really remember it all that well but i'm pretty sure it lasted around 3-5 seconds, I was really scared so I attempted to sprint underwater. The only thing I could really think about (All I know about) is that it could have possibly been Angel Shark(s) but like I said it was in the afternoon so I doubt it. Thanks in advanced!