Okay my uncle is a geologist and occasionally has to go diving in certain places for his job. Several years ago he and a few people he works with were preparing to dive in the sea a bit off of Japan called the Challenger Deep when an absolutely huge dorsal fin popped out of the water and circled the boat several times before disapearing. One of the people he was with estimated that the shark had to be 60-70 feet long. Do any sharks grow this big?

No, not that long. Sorry, but the longest shark recorded was the whale shark at about 41 feet long (12.6 meters), although eye-wittness reports say whale sharks being up to 60 feet long. Although I dont know what species of shark youre talking about. It may be possible, but highly unlikely.

i believe dolphins have large dorsal fins 3 times as big as sharks and since u say he only see the fin it might have been a dolphin

maybe it was a whale shark.

Only 1 species of dolphin has a dorsal fin that large, the killer whale. The others are roughly proportionate. Some sharks have larger, some have smaller.

No shark gets or has ever gotten that big. I would seriously doubt the report as visual estimates are extremely inaccurate

Megalodon was that big

but that was a prehistoric shark only aye???

How big exactly. If the fin was about 6ft tall then the chances are that what he saw was a killer-whale (orca). I've attatched a diagram from wikipedia to this post to give you an idea of the fins size and dimensions.

oops! didn't notice you'd suggested that already. Sorry.

Edit: And now my attatchment is attatched to the wrong post, what a day I'm Having.

maybe during prehistoric times

Orca.

It would be wise to use the term no extant shark in this case.

As openmind has suggested perhaps a whaleshark? they grow to 20 metres long & could be found in that location.

I would believe they were, they were huge, do a google search for it then you shall see.