Three fishermen have filmed an incredible encounter with two extremely rare deep sea sharks, nearly 50 kilometres off the US coast.

Val Costescu was fishing with his mates, Andrew Chang and David Stabile, off San Diego in California last Sunday when two shapes approached their boat from the blue.

The trio grabbed their cameras and filmed the encounter, not realising the beasts were megamouth sharks – an elusive deep sea shark.

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"We didn't know what the sharks were at first," Costescu told 9News.com.au.

"As soon as we got the species confirmed we were shocked to learn that we had an incredibly rare experience.

"It was crazy."

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Megamouth sharks were first discovered in 1976, by a US navy research vessel operating in Hawaii.

As such, not much is known about the species.

It's believed the animals can dive to depths of 1000 metres – maybe even deeper – and live up too 100 years.

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Only one megamouth shark has ever been reported in Australia.

It washed up in Mandurah, WA in 1988.