George Bush Commentary (not the former U.S. President): http://archive.org/stream/notescriticalan06bushgoog#page/n245/mode/1up
Adam Clarke: "If the Lord make a new thing - יהוה יברא בריאה ואם veim beriah yibra Yehovah, and if Jehovah should create a creation, i. e., do such a thing as was never done before."
NET Bible: "But if the Lord does something entirely new, and the earth opens its mouth and swallows them up along with all that they have, and they go down alive to the grave, then you will know that these men have despised the Lord!"
Footnote from NET Bible: "tn The verb בָּרָא (bara’) is normally translated 'create' in the Bible. More specifically it means to fashion or make or do something new and fresh. Here the verb is joined with its cognate accusative to underscore that this will be so different everyone will know it is of God."
"But if a death which hath not been created since the days of the world be now created for them, and if a mouth for the earth, which hath not been made from the beginning, be created now, and the earth open her mouth and swallow them and all they have, and they go down alive into Sheul, you will understand that these men have provoked the Lord to anger" (Targum on Numbers: Pseudo-Jonathan).
[From The Targums of Onkelos and Jonathan Ben Uzziel On the Pentateuch With The Fragments of the Jerusalem Targum From the Chaldee by J. W. Etheridge, M.A. First Published 1862.]
Rashi's Notes on Numbers 16:30:
"But if . . . a creation: A new one."
"the Lord creates: to kill them through a death by which no man has died until now. And what is this creation? 'And the earth will open its mouth and swallow them up.' Then you will know that they have provoked the Holy One, blessed is He, and I [Moses] have spoken by Divine word. Our Rabbis interpret it: If there was a mouth already created to the earth from the time of the six days of Creation, well and good, but if not, let God create [one now]. - [Mid. Tanchuma Korach, Sanh. 110a]"