Godzilla’s giant body is completely implausible according to almost every physicist, mathematician, and paleontologist who have gone into depth as to how his dimensions just shouldn’t be possible. His cartilage disks would have to be about 12 times stronger than a normal human and in order for his bones to support his massive frame they would have to be able put on a force that could not only support his weight but also to not be crushed when he goes diving beneath the sea. Popular Mechanics theorizes that Godzilla would have to put forth a force of about 3000 Mega Pascals in order for his bone structure to stop his knee caps from snapping from beneath him.

Source: Godzilla Raids Again (1956)

His tail slithers and slathers behind and in the Heisei Series (The Godzilla films from 1980 onwards) his tail contained a second brain which handled his motor functions. However no reptiles or dinosaurs have ever been known to have contained more than one brain. This myth comes from the discovery of the Stegosaurus by Othniel Charles Marsh who discovered that the Stegosaurus had a canal in it’s tail from which he theorized must of contained a second brain. However, no nervous tissue has ever been extracted from fossilized remains, so this remains a myth.

Source: Gozilla vs. MechaGodzilla