"It was the first time any of the boys had actually seen the sea. Cal and Chester stared at it in silent amazement, a light breeze cooling their sweaty faces."
-Narrator, DeeperThe Subterranean Sea is a massive body of salt water located in The Deeps.
It is believed that the subterranean sea has been in existence since the planet was created. It is also possible that, similar to the theory of the voids being created, tectonic plate movement caused it's creation, opening a crevice on the surface that caused large quantities of water to spill down and form the sea. An island later formed in the middle of the sea.
During the war against the Styx, Will, who had been lost in a group of lava tubes, stumbled upon the beach. Later, he, Chester, Cal, and Elliott traveled through the beach to reach a causeway that led to the island while they were on the run from the Styx. Soon after, the Styx Limiters, with Sarah Jerome and Bartleby, raided the beach and the island to find the children, who had already escaped the island in a small boat. From this boat, Elliott set off a bomb that destroyed the island and killed the majority of the Limiters. Sarah, who had survived, began to chase after the children, who had left the ocean behind.
Located in the Deeps, the sea is set inside of a gigantic cavern, which it takes up the majority of. The rest of the cavern is mostly a beach and jungle. There used to be a large island near the center of the lake before it was destroyed by Elliott.
A small, natural causeway extends from the beach to the island, although traveling this way is extremely dangerous due to the currents of the sea.
The waters are said to be black, partially due to the sediments in the water as well as the darkness of the cavern. These waters are inhabited by prehistoric, carnivorous creatures.
- Night Crabs (also known as Anomalocaris canadensis)
- Devil's Toenails (Gryphaea arcuata)
- A species of Pterosaurs
- Many species of prehistoric fish, insects, and birds.
- Many species of prehistoric flora, including a caffine-like plant that replenishes energy.