Or HaChaim on Numbers 16:31/אור החיים על במדבר ט״ז:ל״א

1 ככלתו לדבר את כל הדברים, "when he finished saying all this," The earth timed its action at the precise moment when Moses completed what he had to say. As soon as Moses was finished it swallowed Korach, etc.

2ותבקע האדמה אשר תחתיהם, The earth which was underneath them split. Why did the Torah have to write both that the earth "split" and that it "opened its mouth?" Why did the Torah describe the earth as אדמה when describing that it split, and as ארץ when "it opened its mouth?" Perhaps the mouth of the earth had not previously been situated exactly where Korach and his associates stood at the time so that the earth's reaction had to be described as a crack in its surface; for practical purposes this now became the earth's mouth. The crack was described as a crack in the אדמה, the mouth as the mouth of the ארץ.