Rashi: It refers to the future, on the day when "veha'Aretz ka'Beged Tivleh" (Yeshayah 51:6). Bnei Korach saw a miracle done for them - all around them was swallowed, and they stood in the air. Via Ru'ach ha'Kodesh, they told Yisrael that a similar miracle will be done for them in the future.

Radak: This is a metaphor, as is "b'Mot Harim", for the strong wars that will be among the nations - "v'Chutesu Goy b'Goy v'Ir b'Ir" (Divrei ha'Yamim II, 15:6).

Radak citing Ibn Ezra: Because the height of the sun moves from level to level and from Mazal to Mazal, a time will come when the dry land becomes sea and vice-versa. Really, this will not happen - but even if it did, we will not fear - "Geval Samta Bal Ya'avorun Bal Yeshuvun Lechasos ha'Aretz" (Tehilim 104:9).

Malbim: The land will change its place.