how did meg survive under all that pressure

Some sea creatures exploit great depths. The biggest physiological challenges in adapting to pressure are probably faced by those animals that must routinely travel from the surface to great depth. Two such animals are the sperm whale and the bottlenose whale.
Diving to depth can result in mechanical distortion and tissue compression, especially in gas-filled spaces in the body. Such spaces include the middle ear cavity, air sinuses in the head, and the lungs. Development of even small pressure differentials between an air cavity and its surrounding tissue can result in tissue distortion and disruptiona condition in human divers known as "the squeeze." In some species of cetaceans, the middle ear cavity is lined with an extensive venous plexus, which is postulated to become engorged at depth and thus reduce or obliterate the air space and prevent development of the squeeze.