In addition to Marquis Fugitive - Viswinti, who worked at the Overmarine Laboratory, and Salty Sea Marquis - Bluposi, who was constructing an overmarine city at the mouth of the Whirlpool River, the remaining five mermaids all dived into the ocean with Liszt, heading for the Undersea Sanctuary – Ach was focusing on researching the devil, so he did not join them.
The location of the Undersea Sanctuary was in the Shell Sea, situated between the Flame Islands and the Third Archipelago.
The mermaids' swimming speed was not slow, and a few hours later, they successfully reached the Undersea Sanctuary. The water depth was approximately eight hundred meters, and the light was somewhat dim, but the sanctuary resembled a massive crystal dome, enveloping an underwater region and shimmering with a faint rainbow glow.