5 miles down the Earth's mantle
The Deeps are an incredibly huge network of caverns, tunnels, and land features beneath the Colony. This is where Colonists are banished to and where many renegades live. It is where many of the events of the Tunnels series take place, especially in Deeper and Freefall.
The Deeps may have been around since the Earth's beginning. Will hypothesized that the Subterranean Sea could have been formed by tectonic plate movement millions of years ago. He also used tectonic plates to guess how the Great Plain and the Pore were formed. The Sharps were probably formed by ages of condensation. Over all these years, the Deeps have still remained a hostile place, where Colonists have been banished to. Sometime in the past, Drake saved Elliott in the Deeps and the two became partners.
In Tunnels, Dr. Burrows takes the Miners' Train to the Deeps from the Colony, as part of his underground adventure. He discovers the Tablet Caves and a miniature Stonehenge. After debating with Imago, Will and Cal decide to take the train to the Deeps too (with Chester).
Deeper focuses primarily on Will, Cal, and Chester's adventures in the Deeps. The trio jumps off the Miners' Train to find the Miner's Station, which they must hide from. Cal shows them a cave with a natural fire, along with mummified Coprolites. They then stumble upon Crevice Town, where they stock up on supplies and rest. They also see Dr. Burrows has been there. Cal then shows Will and Chester the Great Plain, where they see a Coprolite barge. Unknown to them, Dr. Burrows was on the barge in a Coprolite suit. Cal then finds a sugar trap and ends up getting 'killed.' The horrified Will and Chester wander around some tunnels around the Plain, where they see the Limiters executing a group of Coprolites. Drake and Elliott appear, holding them at knifepoint and taking them back to their base. After introducing themselves, Drake and the group heads back to the sugar trap to rescue Cal. The mission is successful, although Cal becomes stoic after the experience. Drake and Elliott teach the boys how to survive in the Deeps, including defense and food foraging. On one patrol with Elliott, Will sees the Limiters have hung Coprolites in a tunnel as a warning to renegades. Also, the Styx have been blowing up the Coprolite villages and purging the Deeps for the Dominion plot. Soon the group is ambushed by Limiters and forced to separate.
Cal, Drake, and Elliott cross the Great Plain to reach the Bunker. The trip inside scares Cal, because dissected bodies and old experiments are everywhere. At the end, the Limiters ambush them and a battle ensues. During the fight, most of the Bunker is blown up, and Elliott's hunch that the Styx are using a virus is proven right. Drake is left behind. Will, who is without supplies, wanders for a day through the dark tunnels of the Deeps until he comes upon the Subterranean Sea. Chester is there, luckily, and the other group meets them. After Will and Elliott put 'Drake' out of his misery, the group makes a camp on the sea's island. Will is amazed by the findings there, but doesn't want to eat the Night Crabs and Devil's Toenails (they are extinct on the surface). When Elliott notices the Limiters have spotted them, they pack up camp and escape using a wood boat. As they sail away, Elliott uses charges to blow up the island. They then row to a rocky, foggy part of the Deeps, where Chester is freaked out by flying lizards. Elliott intends to take them to the Wetlands, a while past the Pore. In a tunnel, Will finds Sarah and learns the true story. Soon the group heads to the Pore, where Elliott realizes they are cornered. The Rebeccas and Styx military appear. In the big climax, Will, Chester, Bartleby, and Elliott are shot down into the Pore. Sarah then pushes the Rebeccas into the Pore in the end.
In Freefall, the Styx search around the Pore to see if the twins or Will and co. survived the fall. The Old Styx sends the Limiters to the Bunker to extract the Dominion virus from the corpses there. However, Drake steals a Coprolite mining machine, drives it across the Great Plain, and uses it to destroy the Bunker.
In Closer, Will and Elliott use the Ancients' staircase passage to reach Smoking Jean. They then follow the crew's tunnel markings until they reach the underground harbor and radio Drake to rescue them. Meanwhile, the Styx deploy hot-air balloons down the Pore to recover the Dominion virus from the inner world. They also set up camp around the rim and build a wood platform expanding over half the Pore. The mission is a success and the virus is returned to the Colony. The Styx also use Coprolite mining machines to drill all the way to the inner world to bring the Darklit New Germanian soldiers up to the Colony.
In Spiral, the Deeps' final appearance, Drake and the group head back underground to seal the inner world off with nukes. They take the Miners' Train down to the station, and then use horses to travel the rest of the way to the Pore. They kill the Styx guards there and drive a Coprolite mining machine into Smoking Jean to reach the inner world.
The Deeps include many caverns and tunnels, which are easy for a person to get lost in. There are also many landscape features, some which are so unusual that even Will doesn't know how they formed. The Miners' Train and its station are located in a cavern near the 'start' of the Deeps. Many passages split off, and others lead to the Great Plain. The Deeps consist of natural features (the Sea, the island, the Sharps), while others are artificial (the Bunker, the Tablet Caves, and Martha's Shack). Elliott and Drake also have a base in the Deeps.
- Cave Cows
- Sugar traps
- Cave Oysters
- Spider Monkeys
- Dust Puppies
- Flying Lizards
- Loop Snakes
- The Great Plain
- The Subterranean Sea
- The Wetlands and The Wastes
- The Bunker
- The Sharps
- The Pore
- Smoking Jean
- Puffing Mary
- Four unnamed voids
- Martha's Shack