What makes shipwreck diving appealing is also what makes it so dangerous. Wrecks, for example, can sink to extreme depths. The Andrea Doria is known as the 'Mt. Everest of diving' partly because it's an intimidating 180 feet below the water's surface. And that's only to the top of the wreck. At such depths, breathing gets very complicated. Unlike on Everest, where oxygen levels are thin, the weight of water at more than 20 feet deep compresses the oxygen that divers breathe. That oxygen density, in turn, wreaks havoc(破坏)on the human nervous system and can lead to unconsciousness and eventually death. Nitrogen also takes on deadly traits underwater. Again, the weight of water is about 64 pounds per cubic foot is what transforms this otherwise harmless gas. As a diver plunges deeper underwater, excess nitrogen is compressed in the bloodstream. If divers rise too fast to the surface, the compressed nitrogen expands and can fizz out of the bloodstream, entering the spinal column and causing an extremely painful and sometimes deadly condition known as the bends. Nitrogen's other underwater effect is a little more pleasant, but equally lethal. It is called the whiskey factor, at every 50 feet of depth it's like taking a shot of whiskey. While an alcoholic buzz may be an appealing sensation on land, losing your senses underwater can lead to fatal accidents. And shipwrecks hold plenty of potential for accidents. Underwater wrecks are nothing more than a tangle of lines and rotting structures that present a deadly maze(迷宫)to divers. One device divers use to avoid getting snared or disoriented in a wreck is a guideline, or coiled string, that they tie along their path. Like a trail of breadcrumbs, the guidelines allow them to retrace their route. Despite such precautions, the captivating(迷人的)underwater world of a shipwreck can make even the best divers lose focus. Equipment in the sport as a whole has also seen great advances in recent years. In the late 1980s introduced wristwatch-sized diving computers that tell divers when it's time to surface and at what rate, better dry suits that keep the body warmer at chilled depths and mixed gasses that replaced compressed air to make the sport less risky. By saying 'The Andrea Doria is known as the 'Mt. Everest of diving'', the author means that ______.