The Gate

This article's plot summary may be too long or excessively detailed. Please help improve it by removing unnecessary details and making it more concise. (April 2016) (Learn how and when to remove this template message)
12-year-old Glen has a nightmare of finding his home abandoned, then going into his treehouse only for it to be struck by lightning and collapse. In his backyard, workers have cut down a tree and unearthed a geode. Glen returns with older friend Terry to investigate. Though the workers attempted to fill the hole left by the tree, Glen and Terry breach the surface, finding a large geode. Glen catches a splinter and leaves a small bit of blood behind.
Glen's parents leave town for three days, placing his 16-year-old sister Alexandra ("Al") in charge. While Al throws a party, Terry and Glen break open the geode after discovering it has left strange writing on a notepad, and read the incantations aloud. They go downstairs just as the party-goers are playing a levitation game. Everyone is shocked when they levitate Glen.
That night Glen sees his bedroom wall stretching, and Terry embraces an apparition of his dead mother that turns out to be Glen's dog Angus, who dies as a result. The next day, Terry brings a heavy metal album to Glen's house with lyrics based on "The Dark Book". He believes the hole in Glen's backyard is a gateway to a domain of evil gods, and speculates that their actions opened it. He concludes that the only missing element would be to deposit a sacrifice into the hole. Unknown to them, a friend of Al's dumps Angus into the hole, completing the summoning. After reading a section from "The Dark Book" that is supposed to close the gateway, the boys find the hole closed and assume their efforts succeeded.
That night, a swarm of moths shatter Glen's bedroom window, and Angus' corpse is found in Terry's bed. Demonic arms try pulling Al under her bed; Terry and Glen barely save her. They try fleeing the house, but are greeted outside by Glen and Al's parents, actually disguised demons. After returning to the house Al volunteers to inspect the yard, but the others see it swarming with small demons and call her back. Terry leads everyone to the basement to retrieve "The Dark Book", but it bursts into flames. They then attempt to stop the creatures by reading the Bible. Terry reads from Psalm 59 and the hole seems to close, but he slips and falls into the hole. Terry is attacked by the demons before Al and Glen pull him out. Terry reads from Genesis, then throws the Bible into the hole, causing an explosion that appears to seal the hole.
That night, a wall breaks open and a construction worker's corpse falls through. The worker pulls Terry into the wall, which seals behind him. Upstairs, Al notices a hazy image of the construction worker in her mirror before Glen bursts into her bedroom. Al throws a stereo at the construction worker and he disintegrates into dozens of little demons. Al holds the bedroom door shut while Glen races downstairs to find their father's gun. A demonic version of Terry appears, biting his hand before Al stabs Terry in the eye. Al and Glen hide in a closet, but the construction worker breaks through an interior wall and drags Al away.
Glen realizes that Terry and Al represent the two human sacrifices that would fully open the gate. He also realizes that the rocket Al intended to give him for his birthday, a symbol of love, light and purity, may stop the rise of the Old Gods. He makes his way upstairs as the floor collapses, revealing a chasm beneath the house. Glen attempts to launch the rocket, but his matches keep blowing out. The wind sucks Glen onto the foyer, where a giant serpentine demon emerges. The demon pats Glen on the head and touches his hand before returning to the hole. Glen discovers the demon's touch has placed an eye in the palm of his hand. He stabs the eye, then struggles to descend the staircase, at which point the demon re-emerges. Glen uses a battery-powered launcher to fire his rocket into the demon, causing it to explode. Angus emerges from the front closet, seemingly restored to life. He is followed by Terry and Al, also unhurt. The kids worry about how to explain the wreckage of the house to their parents.

Question: What is beneath the house?
Answer: A chasm