Seshem.eff Er Aat (Location)
An Isu facility, accessed through the Nomarch's Tomb in the Black Desert (see that location guide here).
It's hard to tell from up top, but the water here hides both a extemporal Messenger pillar, and lots of scattered silica.
Dive down and gather the silica, then engage the pillar (it needs five silica to activate)
Doing this completes the location, and delivers another message to Layla.