Learn Russian: Тот вулкан — самый опасный.

Самый is used right before an adjective to create the superlative degree ("the most"). It behaves like an adjective itself, meaning it must match the noun it describes in gender, number, and case. Because вулкан is a masculine subject, both самый and опасный take the standard masculine nominative ending -ый.

Why is there a dash (—) after вулкан?

In Russian, the present tense of the verb "to be" (am, is, are) is normally dropped. When you link a subject like Тот вулкан directly to the adjective describing it, a dash is often used in writing to visually represent that missing "is".

Why does this sentence use Тот instead of Этот?

Этот means "this" (something physically close to the speaker), whereas тот means "that" (something further away). Since the sentence is talking about "that volcano" in the distance, we use Тот. It is in the masculine form to match вулкан.

If we were talking about a feminine noun like "mountain" (гора), how would this sentence change?

Every word describing the noun would need to change to its feminine form. Тот would become Та, самый would become самая, and опасный would become опасная. The new sentence would be: Та гора — самая опасная.