Your examples are mixing two senses of the word challenging. The entry in Slovar-vocab is wrong when it states "stimulate" is a synonym of "challenge" when it means "something difficult".
- The book I read stimulated me.
- The book I read challenged me.
In sentence 1, the book is thought provoking or inspired interest in a topic.
In sentence 2, the language of the book was difficult and it took a lot of effort to read, or it forced me to confront my thinking about something and it was a little uncomfortable for me to realize that maybe I need to change my opinion or understanding. It's not clear from the context which meaning is the intended one.
One interesting thing to note is that while "stimulating" is sometimes listed as a synonym for "challenging", the opposite isn't usually true. They are only related through the word "provocative" which has two meanings:
- Serving or tending to elicit a strong, often negative sentiment in another person; exasperating.
- Serving or tending to excite, stimulate or arouse sexual interest.
"Stimulating" is provocative in a positive sense, but without necessarily the sexual connotation, like "exciting". "Challenging" can mean provocative in a negative sense, like "defiant" or "disturbing", but it usually means "difficult to do".
Sometimes, not only in English but in all languages, we want to emphasize certain situations. And then the language itself gives us devices that aren't always common, but we use them anyways, based on known and meaningful expressions.
I am very hungry
We can depict that the subject has surpassed the status of just "hungry" for they must have stayed a long time without eating. This is a known meaningful expression.
Murder is very illegal
From this, even if it doesn't make much sense in the binary nature of the word "legal", we can depict that "murder" is a crime that, morally or ethically, has surpassed the status of "illegal".
We can state that by looking at another not-so-serious crime:
Parking on the sidewalk is illegal
Yeah, we all know it is illegal and wrong. But it is a petty crime compared to murder. In some countries murder is penalized with life imprisonment, even with death penalty, while parking on the sidewalk gives you a fine and, in the worst of the cases, your car is towed.
We can still say that "Murder is illegal", of course it is, but in the sentence, the "very illegal" was made to emphasize.
Best Answer
Semantically they're very similar, but syntactically, the difference is that underlying is an adjective, whereas underneath is either a preposition or an adverb.
The syntactic difference means that, like most adjectives, underlying usually occurs before the noun it modifies...
...but note that we can also use it as a "continuous verb form", with "point" as the subject...
Apart from that syntactic difference, note that the relatively less common underlying usually refers to something fundamental, that underpins something else, whereas underneath (and plain under) merely implies something being (physical or figuratively) below something else.