Learn Spanish (Spain): El volcán sería muy peligroso. - The volcano was probably very dangerous.

In Spanish, the conditional tense is often used to make a guess or express probability about the past. While sería normally translates to "would be", here it acts as an epistemic conditional. This means it expresses uncertainty or deduction about a past state, so the sentence translates naturally to "The volcano was probably very dangerous."

If this expresses a guess about the past, how do I guess about the present?

To make a guess or express probability about the present, you use the future tense. You would say El volcán será muy peligroso to mean "The volcano is probably very dangerous." Using the conditional tense (sería) simply pushes that same kind of guess into the past.

Why use a form of ser (sería) instead of estar (estaría) for "dangerous"?

In Spanish, we use ser to describe inherent traits and characteristics. Being dangerous is considered a fundamental characteristic of a volcano, rather than a temporary condition or mood, so ser is the natural choice here.