Megamouths are known to get up to 17 feet long, can live up to 100 years, and flash their roughly 50 rows of small teeth as they glide underwater. However, it isn't considered a threat to humans, because its classified by the Florida Museum of Natural History as a "filter feeder," subsisting mostly off of krill and other small prey it can easily catch. They typically swim deep -- cruising as far as a mile below the water's surface, and have been spotted primarily off the coast of Japan and Taiwan.
The Museum also notes that they're characterized by having a "soft, flabby body and poor swimming skills," so maybe Michael Phelps has a chance against one of these guys next year.
h/t CBS News, National Geographic