The most plausible theory is that they have developed a secret method of locomotion, which they are able to conceal from human eyes.

最可信的理论是， 他们已经开发出一种秘密的移动方法，可以将其隐藏起来不被人眼看到。

If the book was to be a success he must have 'adventures', and an unusual method of locomotion was the easiest way of providing them.

如果这本书要成功， 他必须有“冒险” 经历，而一种不寻常的移动方式是提供冒险经历的最简单方式。