Created at Twilight

"But if Hashem will create a phenomenon, and the earth opens its mouth and swallows them" [Korach 16:30]

Rashi cites the Gemara in Maseches Sanhedrin [110a]: "If the mouth of the earth is a creation from the six days of Creation," said Moshe Rabbeinu, "then fine. But if not, may Hashem create one!"

Why, asked the Chasam Sofer [R' Moshe Sofer], did Moshe Rabbeinu have any doubt as to whether or not the mouth of the earth had been created during the six days of Creation?  For Chazal state [Avot 5:9] explicitly:

Ten things were created at twilight on the eve of the first Sabbath:

the mouth of the earth

the mouth of the well

the mouth of the ass

the rainbow;

the manna;

Aaron's staff;

the Shamir, writing;

the inscription on the tablets of the Ten Commandments;

and the tablets themselves. 

How then could Moshe have overlooked this Mishnah?

One of the ten things enumerated by Chazal, answered the Chasam Sofer, was "Moshe's grave".  This grave, whose location is unknown, was also created at twilight of that erev Shabbos.

Hashem therefore concealed all ten things from Moshe so that he would not hear of his own passing.  Thus, Moshe had to request "If the mouth of the earth is a creation from the six days of Creation, then fine. But if not, may Hashem create one!"

Source: Rabbi Yisrael Bronstein