A very lucky group of divers were exploring the crystal blue waters off the coast of the Polynesian island of Tonga when they were startled by a once-in-a-lifetime encounter.

The amazing underwater video was taken by Australian diver, Scott Portelli. The divers believed that they witnessed a type of dolphin called the false killer whale.

The divers were actually looking for humpback whales, when the curious pod of false killer whales swam up from the depths.

"This mother and calf False killer whale rolled around under us whistling and checking us out. Some beautiful encounters with some rarely seen cetaceans in Tonga," said Portelli.

The false killer whales can grow up to 17 feet and actually don't look like killer whales. They are dark gray, or black.

They get their name from the way they hunt. Like killer whales, these dolphins sometimes attack small whales and dolphins.

Watch the amazing footage below!

Have you experienced a surprise like this on your vacation? Let us know in the comments below!