The objective to find and, hopefully, destroy the source of Voldemort's immortality and power sends Harry and Dumbledore on a perilous journey to an isolated cave set deep within a windswept cliff. Stuart Craig recalls, "When we were doing our location scouts, we found the Cliffs of Moher in the west of Ireland and immediately thought they would be spectacular for the entrance of the cave."

The filmmakers wanted the cave interior to seem vast beyond measure, "so there was no possibility of building that physically," Craig acknowledges. "Apart from the point at which Harry and Dumbledore first arrive and the island formation on which everything inside the cave happens, the set is entirely virtual, designed in the computer. We'd had our first totally virtual set on the last film, so we approached this one with a bit more confidence."

The interior of the cave appears to be made up of geometric crystal formations, so Craig and his team began by researching various rock formations. They also visited a quartz crystal cave and a salt crystal cave to study the properties of crystal surfaces. "Reproducing salt crystals requires some major R&D and a lot of experimenting with synthetic resins," the designer remarks. "The challenge was to create something that was magical but credible at the same. It was also a challenge for Tim Burke and our visual effects team and also for (director of photography) Bruno Delbonnel. How do you light this glowing crystal island in the middle of a huge black void? Everybody was part of the process; it was an exciting collaboration."

Within the recesses of the cave, Harry and Dumbledore face terrible dangers as, for the first time, Harry is forced by Dumbledore to take control. Radcliffe states, "When Dumbledore takes Harry on this mission, it is sort of a rite of passage. He is initiating Harry into what he is ultimately going to have to do. This is the beginning of the story of defeating Voldemort, and Harry would never shirk that responsibility."

And, whether or not they succeed, new and even more terrible threats await them at Hogwarts.

Heyman reflects, "There is a line in the film where Dumbledore says, 'Once again, I must ask too much of you, Harry.' Does he feel guilt? I don't know if he feels guilt, but he knows that Harry is growing up, and I think this is all part of the education of Harry Potter. Dumbledore knows that ultimately Harry is going to be the one who has to face Voldemort, and in introducing him to Slughorn and exposing him to some of Voldemort's history and taking him on the journey that he does, he is preparing Harry for the final battle that lies ahead."