Written in 1973, a massive 50 kilometre long alien cylinder begins to pass through the solar system provoking a hurried effort to intercept it. The closest available ship rushes to rendezvous so as to have a quick study before it gets too close to the sun. Able to enter via an airlock on one end of the ship, the crew explores the huge world found inside, a world full of wonder and mystery. As usual, the science is spot on. This is the best novel of Clarke's since 2001 and Childhood's End and is a truly grand adventure full of puzzles and ideas that lead you asking more questions than are answered. Enough questions in fact to lead to numerous inferior sequels, but enough answers to leave you satisfied. Don't pass up this gem of hard science fiction.