Various citizens of Port Charles have been to Cassadine Island.
The island is first seen in 1981, when the Quartermaines, the Cassadines, the Spencers, and the W.S.B. were all looking for the Ice Princess - a large uncut diamond whose base contained the formula for a powerful weapon. They went to an Underground Bunker located on Cassadine Island and owner Mikkos Cassadine revealed he had plans to build a weather machine that would produce carbonic snow and freeze the world. He would first threaten world leaders by freezing Port Charles as an example to the rest of the world. B-actress Tiffany Hill was also on the island, as Cassadine brother Victor Cassadine's girlfriend. She was repulsed by this plan. She ran into then W.S.B. Agent Robert Scorpio and she decided to help him, Luke Spencer, and Laura Webber stop Mikkos. Meanwhile, back in Port Charles, the temperature was dropping and a blizzard took place. Port Charles was declared a disaster area.
Robert sneaked into Mikkos' bunker, with Luke and Laura following. They set off an alarm, and as they were running to escape, they came upon an ice chamber, with frozen bodies in it, including Alex Quartermaine and Tony Cassadine. They were then captured, and brought before Mikkos, where they were reunited with Robert, whom had also been captured. Luke told Mikkos he would help him, and said he would convince the W.S.B. to surrender. But, during a radio transmission, Luke started to give away their location, at which point Mikkos angrily turned off the radio. There was a scuffle and Mikkos attempted to throw Luke into the ice chamber, but Mikkos was the one who ended up going in and dying. Luke was then able to turn off the machine, saving the world.
In October 2009, Valentin Cassadine brutally took over the Cassadine Island in Greece. Valentin overpowers Helena Cassadine taking over the family's private island in Greece and he slowly poisons Helena.
On July 11, 2016, Valentin is seen lurking around on Cassadine Island. He spies on "Jason Morgan", Sam McCall, Ava Jerome, and Nikolas Cassadine. "Jason" soon realizes that someone else is on the island. Valentin is seen looking at a book of Cassadine lineage before he knocks over a vase. "Jason" and Sam lure him out by falsely proclaiming that they are heading to bed.
They corner Valentin and he claims to be a fisherman named Theo, who was forced to come ashore due to the storm. Sam and "Jason" decide to leave him be, as long as he does what they say. Valentin continues to play his role and goes to warm his hands by the fire. As he is warming his hands, he rolls up his sleeve and you can see the Cassadine family crest tattooed on his forearm.
On July 16, Theo blew his cover and he held a gun on everyone present. On July 18, it was confirmed that Theo is in fact Valentin. On July 19, Valentin forces Nikolas to hand over the Cassadine family fortune to him. As Valentin tries to shoot Ava, Nikolas attacks him, but ends up getting shot and falling over a balcony.
On July 22, Ava seduced Valentin, and then hit him in the head with a book. When Laura Webber lashed at Valentin for shooting Nikolas, Valentin went to shoot her, but Kevin Collins stepped in the way and was shot instead. On July 26, Valentin is forced to flee after his captives get the upper hand. He escapes into the tunnels and orders his men to make sure none of them escape.
Drew later has flashbacks of the time he was being tortured by Helena's henchman on the island while Jake Spencer watched.