When small, Hydras resemble certain species of Salamanders and/or Wyrms. They hatch from eggs buried beneath the ground in large groups, forming sinkholes. They emanate methane, which make it hard to breathe in the holes. After hatching, they crawl up, intertwining their tails so they eventually bind together. They grow extremely fast and can reach very large sizes, forming Wyrm Kings.
The Hydras' only natural enemy are the Giants. The Giants and the Hydras all wake up at the same time, which only happens every 500 years. The Giants move to the sinkholes and eat the tiny baby Hydras before they twine together, using the methane to breath fire. The Giants and the Hydras fight against each other until one of them has slaughtered the other. If the Giants win, they go back to sleep. If the Hydras win, they destroy everything in their way.
The largest concentration of Giants were around Mangrove Hollow, because it was there the Wyrm King was hibernating.Hydras are as big as a giant.
Nick Vargas and his stepsister Laurie, along with Nick's brother Julian, thought that if they didn't stop the Giants, all Florida was going to burn. Therefore, they lured the Giants out to sea with a Mermaid song. However, they soon discovered they actually needed the Giants to stop the Hydras from destroying everything. They used Nixie song to lure the Giants back to Florida land.They then received help from Jack Junior and the Grace children to battle the Wyrm King, although nature was ultimately allowed to have its own way; Nick managed to cut the giant Wyrm King apart, making the many Wyrms confused as it was the first time in centuries they moved individually. Thereafter, the Giants killed all the Hydras and went back to sleep.