The gurus talked about it in the Q &A stream on June 21st, at about the halfway point (at exactly 14:06 in the video replay). There is a secret spot called Mermaid's Cove somewhere on Mua Pel'am, the natural island, which it sounds like is unlockable in only a very specific way. You have to maximize the conservation level of the island with a discovered mermaid actually in your household, and then you will be able to find the entrance to this secret cove. Its main property is that it will spawn NPC mermaids. It is not in the same place as the main cave but is deliberately hidden in a very sneaky spot; in the video the gurus are looking for it all around the volcano at night but never actually find it. I couldn't tell from the conversation whether this cove is an actual hidden lot your sim can travel to, like Sylvan Glade or Sixam, or another rabbithole which, once unlocked, merely triggers a new spawner. I hope it is the first.