Fathom Stone
From WoWWiki
The Fathom Stone is a small pillar in the underwater section of Blackfathom Deeps just before Twilight Lord Kelris's chamber.
Objective of
It contains [Fathom Core] for the quest 
[27] Amongst the Ruins.
Notes
The stone is located in the same location as Baron Aquanis. He will aggro when you access the Stone. Just before Kelris's room, there is a darker colored circle on the map, on the path to Kelris's room. Drop into the water, and the stone is just below this darkened circle.
