What Stalks The Deep by T. Kingfisher

Here is a summary with spoilers for What Stalks The Deep by T. Kingfisher

Alex Easton has had a lot of adventures, but one they don’t want to have is raising a child, which is what their sister really wants them to do. So when Alex and Angus receive a letter from an old friend, Dr. Denton, asking them to visit America and help with a mystery, they jump at the chance and make their way to rural America.

Upon arrival, they meet up with Dr. Denton, who tells them he has inherited an abandoned coal mine. His cousin Oscar went to examine the mine and sent letters about it, saying it was very interesting. However, the letters grew progressively worse. Oscar began describing a red light in the tunnels and an eerie feeling. Then the messages became increasingly disturbing, turning into frantic ramblings, until finally ending with a warning not to visit. After that, no more letters came. Despite this, Alex, Angus, and Dr. Denton head into the mine.

During their time in the tunnels, they encounter many obstacles, including gutted bodies, the strange red light, and poisonous gases that make them light-headed. Eventually, they find Oscar. However, something is off—he isn’t able to speak. When he removes his goggles, they realize it is not Oscar at all, but a creature posing as him. They call him non-Oscar.

The being explains that he and his people live in the tunnels. They have been cut off from the outside world and wish to remain that way. However, in recent years they have felt the ground shifting and people intruding into their home. They are unsure whether to prepare for an invasion or simply hide. The group brings non-Oscar with them as they continue deeper into the mines.

As they explore further, they discover there are not only non-Oscar’s people in the tunnels but other, more violent creatures as well. This is how they learn that the real Oscar is dead. The group manages to seal off the violent creatures, and Dr. Denton negotiates a deal with non-Oscar and his people. Someone from the human world will always watch the tunnels to ensure no one enters to harm them. In turn, one of non-Oscar’s people will also stand guard, to ensure their safety and to keep their secret.

Alex decides they have had enough of America, but they stay with Angus a bit longer to do some sightseeing before returning home.