Some place with a megaladon big enough to swallow the entire party without any effort. The megaladon doesn't have to be anywhere near them, but the sight of which just sounds like a good scare. This could be the "pet shark" of just about anything else, or the characters could have been shrunk down to tiny size when they passed through the portal, or the whole thing could be an illusion, or any of the above (or below) options. I just like giant sharks.