During the scene where the 1920's mine shaft is discovered, a rusty cage with a bird skeleton is visible briefly. In the early days of mining coal, canaries were lowered down to test whether or not there were toxic gasses in the air. If the canary died, it would mean that there was a dangerous substance in the air. Since this method of testing is considered extremely cruel to the canary in this day and age, modern automatic digital/electronic testers are used. Some of these testers are even yellow and shaped like canaries as a joke.
The mine in Beneath isn't a real coal mine, it's a set that was designed to mimic a real closed-pit coal mine shaft because filming in a real mine, especially when having to carry down film equipment, is unsafe and time-consuming, not to mention expensive.