The Latin source of the word marvel is mirari, "to wonder at," and that's exactly what marvel means when it's a verb. You may marvel at the depth of the Grand Canyon or at the size of a pimple.
When it entered the English language in 1300, marvel meant "miracle" –- that is, a supernatural act of divine agency (miracle actually comes from the same Latin word as marvel). By 1386, Geoffrey Chaucer was able to use the Middle English meruailles to describe more mundane astonishment. Chaucer also used marvel –- maruaylen, actually –- as a verb. In contemporary usage, the close synonym wonder is used much more often –- unless you happen to be a fan of the comic book universe created by Stan Lee. As a noun, anything that provokes wonder is a marvel.