This is the greatest dungeon music ever composed.
The Undersea/Ocean Palace actually isn't Chrono Trigger's Very Definitely Final Dungeon; that's the Black Omen/Black Dream (#36 on this list). But it sure as hell feels like the final dungeon. So did Magus's Castle; Chrono Trigger really threw some epic dungeons at you. And this is, apologies to Magus's Castle, the most epic of them all.
Actually, the setup is almost the same as Magus's Castle: Race to the arch-villain and hope you get there in time before the power-crazed idiot unleashes Lavos a few thousand years too early. But while the build-up to Magus may have been more elaborate, the build-up to the Ocean Palace and Queen Zeal is much more intense. The pace of the game picks up dramatically when you reach Zeal Kingdom in 12,000 B.C., and doesn't really stop until after your meeting with Lavos at the Ocean Palace.
And when you arrive at the Ocean Palace, you get punched right in the nuts by this music. Get your ass in gear, son, this music yells at you, drill sergeant style. You either haul ass to the end of this eerie nightmarish temple NOW or the world dies. Go go GO! And so you do. There's a save point at the beginning of the dungeon, which is itself unusual and another clue that this is a manly dungeon you're about to venture into. And just in case you haven't gotten the message yet, when you save the file is titled Lavos Beckons.
And the programmers knew exactly what awesome music they had on their hands here. It is so badass that it does not stop for battles (unique in the game), not here and not in the other place in the game it appears. You see, near the game's end, a scenario arises where the circumstances are desperate, you are the only hope and you have only minutes to get what has to be gotten, with numerous monsters opposing you. So the game designers thought about how to set that atmosphere just so, for about ten seconds, and then said, "set the whole thing to Undersea Palace and don't stop it for battles. Done." And they were right. This music is badass.