Music is a powerful tool in a game.
And I remember that this zone always gave me the creeps, if not frightened me every time. 'Fossil Trench' was a deep, gray subduction zone trench based level that occured in stage two. It had a graveyard look, was dark, and had the most chilling simplistic music. When I was little, I always shook a little. And then you realized that the Trench had an extension after you passed the two sectors... a THIRD, 'abyss' level. In which your trusty helpful dolphin with the oxygen tank couldn't reach. Fossil Trench had several tracks, obviously a favorite of the design team. But I could never forget that very simple bass theme. 'Dun-dunnnnnnn DUNUN.' Even ten years later.
Here's that track, extended a touch with intro and outro, and my take on setting the mood. I added a touch of effect. The sonar, to make you realize it was dark, and you needed something to see, and the motor of your little yellow (orange) submarine.
If you get chills, it worked.
Here's the original track on youtube for your amusement.