Long distance swimming is pretty scary in its own right. Just you and the open water with hopefully a couple of spotters. Now imagine that you're on the high seas and there's a shark right under you.
This is what happened to Adam Walker, who spotted a great white shark while swimming near New Zealand. We can't see the shark in the video, but we do have footage of the pod of dolphins who swam alongside Walker for over an hour, according to the video's description on YouTube.
It makes for a pretty amazing GIF.