The Great Barrier Reef is perhaps the ultimate travel bucket-list destination. Off the coast of Queensland, in northeastern Australia, the underwater wonderland and UNESCO World Heritage Site stretches for approximately 1,500 miles in the Coral Sea. Here are our favorite hotel home bases for exploring the largest coral reef in the world -- adult-only resorts, apartment complexes, and luxury hotels in Cairns (considered the gateway to the Great Barrier Reef), Port Douglas, Kewarra Beach, and the Whitsundays.