My wife and I swam out to unknown fins breaking the surface in Hawaii. In hindsight, we just laugh... WTF were we thinking... turned out to be a herd (?) of huge manta rays. We got right in with them- within inches at times. Their wingspans were 6-8 feet or more. The POWER in their wings is astounding. It was like being among cows- you know they could stomp the [bleep] out of you if they wanted to, but they didn't want to... awesome experience.
Once the peeps on the beach saw us get not-eaten other folks swam out. Amazing experience.
