Numbers 16:25-33 (RSV) Then Moses rose and went to Dathan and Abi’ram; and the elders of Israel followed him.  And he said to the congregation, “Depart, I pray you, from the tents of these wicked men, and touch nothing of theirs, lest you be swept away with all their sins.”  So they got away from about the dwelling of Korah, Dathan, and Abi’ram; and Dathan and Abi’ram came out and stood at the door of their tents, together with their wives, their sons, and their little ones.  And Moses said, “Hereby you shall know that the LORD has sent me to do all these works, and that it has not been of my own accord.  If these men die the common death of all men, or if they are visited by the fate of all men, then the LORD has not sent me.  But if the LORD creates something new, and the ground opens its mouth, and swallows them up, with all that belongs to them, and they go down alive into Sheol, then you shall know that these men have despised the LORD.”  And as he finished speaking all these words, the ground under them split asunder;  and the earth opened its mouth and swallowed them up, with their households and all the men that belonged to Korah and all their goods.  So they and all that belonged to them went down alive into Sheol; and the earth closed over them, and they perished from the midst of the assembly.
The Scottish evangelical Protestant Egyptologist and archaeologist Kenneth A. Kitchen (b. 1932), is author of the magisterial On the Reliability of the Old Testament (Grand Rapids and Cambridge: William B. Eerdmans Publishing Company, 2003). He commented on this incident as follows:
Moses was faced with more rebellion, from Korah, Dathan, and Abiram (Num. 16), as Israel traversed the Arabah rift valley between the Dead Sea and the Gulf of Aqaba. The rebels came to a sticky end when suddenly the earth, as at Moses’ word, swallowed them up . . . At first blush, these are the kinds of reports that attract modern skepticism; but a closer look . . . has suggested that this narrative reflects a phenomenon that could only have been known to someone who knew the local conditions in parts of the Arabah. There exists there kewirs, or mudflats. Over a deep mass of liquid mud and ooze is formed a hard crust of clayey mud overlying layers of hard salt and half-dry muds, about thirty centimeters thick. Under normal conditions one may readily walk over or stay upon the crust without any problem, as if on firm ground; but increased humidity (especially with rainstorms) causes the crust to soften and break up, turning everything into gluey mud. (pp. 191-192)
Another natural possibility is an earthquake. In the article, “Seismic behaviour of the Dead Sea fault along Araba valley, Jordan” ( Geophysical Journal International, Volume 142, Issue 3, September 2000, Pages 769–782, the authors state:
The Dead Sea fault zone is a major left-lateral strike-slip fault. South of the Dead Sea basin, the Wadi Araba fault extends over 160 km to the Gulf of Aqaba. The Dead Sea fault zone is known to have produced several relatively large historical earthquakes. . . .
[W]e suggest that the Dead Sea fault along the Araba valley should produce an Mw 7 earthquake about every 200 years on average . . .
Or it was simply a supernatural miracle, directly brought about by God. That scenario is always possible in the biblical view, as are natural events that come about at just the right time, by God’s providence, or a combination of the two. So, for example, right after the above Bible passage, we have: “And fire came forth from the LORD, and consumed the two hundred and fifty men offering the incense” (16:35). That could have been supernatural fire from God or natural lightning, in a storm that would have also have naturally produced the collapse of a mudflat (assuming the people involved were near same).
The Hebrew word here, esh (Strong’s 784) is used, or possibly used with the meaning of “lightning” in 2 Kgs 1:14; Job 1:16; Ex 9:23-24 (“thunder and hail, and fire ran down to the earth. . . . fire flashing continually in the midst of the hail”: RSV); Ps 18:13-14; 148:8; and other passages, according to the Brown-Driver-Briggs lexicon.
Summary: What caused this startling “judgment” event recorded in Numbers 16, with the earth swallowing sinners? It could have been a mudflat, an earthquake, or a pure miracle from God.