You are Catherine Dubont. The Laser Drill opened a cavern 10 miles beyond the surface of the earth. They installed an elevator which you and some others took to go down all the way into the world of the unknown. As you almost reached the bottom of the drill hole, the elevator rope broke and the cabin dropped down the last 100 feet. Everyone is dead. All lights are gone. The only thing you have left is your modern sonar scanner.
The main thing to know about Sonar is that it's pretty damn intense for a game, that looks the way it looks. Hell, it's pretty damn intense period. One of the best horror games of the year imo.