Read the excerpt from "Protecting the Great Barrier Reef." Its stunning natural scenery makes the Great Barrier Reef a popular tourist destination. About two million people visit the reef each year. Most of those visitors are domestic tourists. This is largely due to the fact that Australia is so far away from most places. There are lots of different ways to experience the Great Barrier Reef. Visitors can go diving, fishing, swimming, and enjoy water sports. Some people choose to tour the reef by airplane or helicopter, and others may tour the reef by boat. Tourists can take in the spectacular scenery from a small dinghy or a grand superyacht. There are even glass-bottomed boats that let passengers view the underwater wildlife and scenery. Adventurous visitors may seek out one of the handful of havens where they can swim with wild dolphins. One of the major appeals of these activities is that most of them do not require special training or expensive equipment. There is no one way for people to experience the reef. Regardless of the activities people choose, they encounter some of the most stunning maritime scenery on the planet.

What evidence shows why the reef is inaccessible to international tourists?