What could ark 2 be?

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I just finished reading Stephen Baxter's "Flood", a rather unlikely scenario where the sea level on Earth rises far more than can be attributed to global warming, glacial melting, etc. The story goes that there are subterranean oceans between the crust and the mantel and that for some unknown reason, there are cracks in the crust and the water starts coming to the surface. Large areas of land are flooded, people are displaces, it's not looking too good. The US and other governments retreat to high ground. Their scientists project that, worse case scenario, the sea level is going to rise to the point that it will cover all the land. Yep, all of it. So they take the time that they have left and use the technology that's available and they build arks. Ark 1 is a space ship that takes off with a crew of scientists that are young enough to bear children. They head off, presumable to another start to start a colony. Ark 3 is a huge ocean liner modeled after the Queen Mary. There's genetically engineered algae that can grow huge rafts for people to live on the surface of the ocean. At the end of the book, possibly leaving open the possibility of a sequel, the question is asked, "what is Ark 2"? I think maybe it's an underwater city. What do you think Ark 2 is?