Green Island, Australia - The world's largest crocodile in captivity has died at the age of more than 120 years old while being held in captivity on an island off Australia's east coast.

Held at Marineland Melanesia Crocodile Habitat on Green Island, off the coast of Cairns in the Australian state of Queensland, Cassius was 18 feet long and weighed well over a ton.

As late as last year, it was reported that Cassius had continued to grow at a surprising rate, stunning scientists with his longevity.

He died on Saturday after suffering from declining health since October 15. At his time of death, Cassius held the Guinness World Record for being the world's biggest crocodile held in captivity.

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In a post on Instagram, Marineland Melanesia Crocodile Habitat announced Cassius' passing, revealing that he had resided in the wildlife sanctuary for more than 50 years and unveiling the story behind his last few weeks.

"It is with deep sadness that we share the passing of our beloved mate Cassius," the sanctuary, which goes as @green_island_crocs, said in a statement.

"He was more than just a Crocodile he was a cherished member of our family and brought joy and companionship to his best mate George for over 37 years."

It was revealed that Cassius had been moved to Cairns on the mainland on the 5th of October to an assisted care residence. He had then suffered a steep decline in health since the 15th, before passing away on Saturday.

"Cassius will be deeply missed, but our love and memories of him will remain in our hearts forever," the statement read.

"Thank you to everyone who visited Cassius throughout his life and offered kindness along the way."