The ancient Greeks believed that Nisyros was created during the legendary "Battle of the Giants", also known as “Gigantomachy”.
It was Poseidon's task to put the Giant Polyvotes out of sight, who, terrified, crossed the Aegean Sea to save his life. Poseidon caught up with him near the island of Kos. With his trident, he tore off a part from the island of Kos, slung it against Polyvotes and finally crushed the Giant and buried him beneath this spot. This part of Kos became the island of Nisyros.
The volcano was created at the spot where the mouth of the Giant was lying. Any time the volcano is active, it's by reason of Polyvotes, who sighs because he’s a prisoner.