Al sonoro rugir del cañón.

This line means, “At the sonorous roar of the cannon.” It completes the chorus by connecting the trembling earth to the sound of artillery. The anthem uses the cannon as a symbol of war, defense, and national strength.

Sonoro means “resounding” or “loud and ringing.” Rugir means “to roar,” a verb often used for animals, storms, or engines, so rugir del cañón gives the cannon an animal-like power. Cañón is “cannon.” Together, the line is not subtle: it is meant to sound loud, martial, and dramatic.