Per Proverbs 30:21-22, which is one of the things under which the earth trembles?

Answer: A servant who becomes king

The four: a servant who becomes king, a fool filled with food, an unloved woman who marries, and a maidservant who supplants her mistress.

“Under three things the earth trembles, under four it cannot bear up: a servant who becomes king, a fool who is filled with food, an unloved woman who marries, and a maidservant who supplants her mistress.”— Proverbs 30:21-23 (BSB)