While it's presently unclear why we're going back to the seafloor, Bismarck's air bubble and the phantom Amaurot are both lingering effects; getting back down there is not difficult.
It looks like Anamnesis Anyder is a deeper part of Amaurot, one with less of Emet-Selch's glamour overlaying the ruins. Or maybe Ancient technology and architecture really are that durable. (12,000 years on the seafloor tho? Hard time believing it...)