Amongst the Ruins
Take the Fathom Core from the Fathom Stone inside the dungeon and bring it to Je'neu.
Take the Fathom Core from the Fathom Stone inside the dungeon and bring it to Je'neu.

Taking the core summons Baron Aquanis, whose drop starts the next quest.
Horde: Je'neu Sancrea in Ashenvale (11.6, 34.3).
Horde: Je'neu Sancrea in Ashenvale (11.6, 34.3).
experience only, 6,550 XP.
Amongst the Ruins is a level 27 quest; you can pick it up from level 21. It takes you into Blackfathom Deeps.
All Blackfathom Deeps quests on one map ·Blackfathom Deeps dungeon guide
Sources: Quest data from ForeverDB.net · ForeverDB: Blackfathom Deeps · ForeverDB: Every dungeon quest for the level 20 to 30 dungeons · ForeverDB: The Orb of Soran'ruk · ForeverDB: Seeking the Kor Gem · ForeverDB: Baron Aquanis · Warcraft Wiki: Blackfathom Deeps (Classic)