Learn French: Il est probable qu'ils entendront l'éruption. - It is probable that they will hear the eruption.

Expressions of probability like il est probable que state a strong likelihood, which French treats as a certainty, so they take the indicative. You only use the subjunctive for things that are doubtful or merely possible.

How would this sentence change if I wanted to say 'It is unlikely that they will hear the eruption'?

You would use il est peu probable que (it is unlikely that). Because this expresses doubt or improbability, it triggers the subjunctive, so the sentence would become Il est peu probable qu'ils entendent l'éruption.

Does the il in il est probable refer to something specific, like the volcano?

No, this is an impersonal expression. Just like the English 'it' in 'it is probable', the il here doesn't replace a noun. It simply acts as a dummy subject to hold the sentence structure together.

If we already knew we were talking about the eruption, how would we say 'It is probable that they will hear it'?

You would replace l'éruption with the feminine direct object pronoun la. It goes directly before the verb it belongs to, and contracts because of the vowel, becoming Il est probable qu'ils l'entendront.