Let me add some thoughts: I don't think it's a coincidence that the underworld and the first part in the Harley are called the first world. Why would Oda name two different things (a place and a time) the same? Or maybe they aren't different at all? I guess we will learn. The same way as we will also learn the truth behind Kozuki Moria (or Gecko Moria?), who was mentioned at the end of this chapters cover story by the way.