Maybe I'm missing something, but I noticed that starting in Part III (Tremors), the story takes us to Hawaii with Dirk Pitt and Summer Pitt. Then the story takes us back to Mongolia with Dirk Pitt and Al Giordino.
How can Dirk Pitt be in two places at the same time? Unless I am missing something obvious, this has been a curiosity for me to figure out.
Read Valhalla Rising, it will all make sense to you (ha!). The Dirk Pitt in Hawaii is Dirk Pitt Jr., our intrepid hero's son. Summer Pitt is Dirk Jr's twin sister. Usually we can say that you don't have to read the books in any particular order, but you have to have read VR before reading any of the subsequent books (Trojen Odyssey, Black Wind, Treasur of Khan) to know who some of the characters are. Check out the "Books- Clive Cussler; Dirk Cussler" category and the "Clive Cussler Book Reviews" category for lots more discussion on that.