Rashi on Genesis 7:11/רש"י על בראשית ז׳:י״א

1בחדש השני IN THE SECOND MONTH — Rabbi Eliezer said, “This is the month Marcheshvan”; Rabbi Joshua said, “This is the month Eyar (Rosh Hashanah 11b).

2נבקעו WERE BROKEN UP — so that they might give forth their waters.

3תהום רבה THE GREAT DEEP — Measure for measure: they had sinned with רבה רעת האדם “great was the evil of man” (i. e. their sin is described by the word רבה “great”) and they were therefore punished by תהום רבה “the great deep” (i.e. the expression “great’’ is used of the instrument by which they were punished for their great sin) (Sanhedrin 108a).