Underdark Southern Tunnels Overview - Baldur's Gate 2
The Underdark's Southern Tunnels are best known as the Beholder Tunnels since the area is filled with Beholder enemies. I strongly recommend you proceed with extreme caution in this area as Beholders can be completely nasty enemies for those who are unprepared. Using items like Cloak of Mirroring or Shield of Balduran will make the Beholders significantly easier.
While you're exploring this area I strongly recommend you hold down the TAB button (or whatever it is on the device you're playing on) that highlights containers you can interact with. Throughout these tunnels there are going to be numerous crevices that are hidden in the walls that you'll only ever find if you hold down TAB.
Entrance Room: Upon entering these tunnels you will be immediately attacked by a group of Beholders. Defeating them will net you Amulet of Spell Warding, Blade of the Equalizer and Eyestalk of an Elder Orb. The Eyestalk of an Elder Orb is involved in The Dragon Eggs quest in Ust Natha and the Blade of the Equalizer is involved in the Cromwell's Blacksmith side quest.
Drow + Sling: There will be some Drow here fighting a Beholder. Attack the Drow while they're distracted since they're the harder opponents. When the battle is over be sure to loot the Sling of Arvoreen +4 in the crevice in this area.
Amulet: At this location you'll find a Greenstone Amulet hidden in a nook. Much like the advice I have given throughout this whole page - hold down TAB or whatever button highlights all containers to find it.