Learn French: Ces rochers sont tout humides. - These rocks are completely damp.

In this sentence, tout is an adverb meaning "completely," and adverbs normally do not change form to agree with nouns. The special, quirky rule for the adverb tout is that it only changes to agree with feminine adjectives that start with a consonant or an aspirated 'h'. Since rochers is masculine, tout stays strictly invariable here.

Do I pronounce the 't' at the end of tout before humides?

Yes, you do! The 'h' in humides is a "mute h" (h muet), which means the word behaves as if it starts with a vowel. Because of this, you must make a strong liaison between tout and humides, pronouncing the 't' so it sounds like "too-too-meed".

What is the difference between rochers and pierres?

Un rocher refers to a large rock, boulder, or crag—the kind you might climb on at the beach or see on a mountain. Une pierre is a stone, which is generally smaller (something you can hold in your hand) or refers to the building material itself.

Could I just say très humides or complètement humides instead?