Underwater City

The City of Caverns is a Sahuagin city built on platforms in the middle of the Sea of Swords. The city is ruled by King Ixilthetocal, although the city is currently in a civil war between the King and the Prince, Prince Villynaty who wants to overthrow the King. The Sahaugin there used to have clashes with the Drow in the Underdark, and there is still a tunnel leading from the City to the Underdark.

Baldur's Gate II
If you accepted Saemon Havarian's way out of Spellhold, the ship you use to get back to Amn will be overrun by Githyanki and then Sahuagin who will take you to their City. Once there you will have to side with either the King or the Prince and kill the other. Once you have done that, you will be given a rope that will allow you to go down the tunnel to the Underdark.