(יב) וַֽיְהִ֥י הַגֶּ֖שֶׁם עַל־הָאָ֑רֶץ אַרְבָּעִ֣ים י֔וֹם וְאַרְבָּעִ֖ים לָֽיְלָה׃
(12) The rain fell on the earth forty days and forty nights.
Wouldn't only a moment of rain have been enough to nullify all existence? Why did it rain 40 days and 40 nights?
After the number seven, 40 is the most frequently seen number in the Tanach. In ancient texts 40 indicates a period of transition or transformation, profound internal change, a new beginning. The inner meaning of 40 is the ascent from one madrega to the next higher one.
Moses fasts for 40 days at Sinai. Jesus fasts for 40 days in the wilderness. the Quran was revealed to Muhammed when he was 40 and received the call to become a prophet. Siddhārtha Gautama sits under the bodhi tree for 40 days in meditation. 40 days are given to Nineveh to repent. Shams teaches Rumi for 40 days in seclusion. 40 days for the spies to see the land. 40 weeks and then birth. 40 years for the journey to the land of promise. 40 days from the first day of Elul until the miracle of Yom Kippur.
40 days of flood and 40 se'a of water in the mikvah.
וְכַמָּה הֵן — אַמָּה עַל אַמָּה בְּרוּם שָׁלֹשׁ אַמּוֹת. וְשִׁיעֲרוּ חֲכָמִים מֵי מִקְוֶה אַרְבָּעִים סְאָה.
(Discussing how much water is in a mikvah) And how much water is that? It is a cubit by a cubit by the height of three cubits. And the Sages calculated the volume of a ritual bath and determined that the waters of a ritual bath measure forty se’a.
R. Schneuer Zalman of Liadi teaches:
The Flood is a cleansing process. The waters of the Flood are (like) the waters of a ritual bath—a mikvah—where the waters themselves do the work of spiritual renewal. The world is reborn, a new course of hope and purpose. And just as a mikvah must contain 40 se'a (about 150 gallons), so did the rains of the Flood last for forty days. One immerses completely in the waters of a mikveh nullifying one's previous state. And by virtue of this complete immersion and self-nullification, we emerge from the waters purified, on a different level, a new being, reborn. (notice the sleight of hand)
R. Zalman teaches:
When a soul is ready to enter Gan Eden, it must first be immersed in the mikvah, River of Light, that flows from the fervent perspiration of the Heavenly Hosts as they sing glory to the Highest.
The River of Light is called guf. The sages declare that the Messiah will not come until all souls have issued out of the guf, the body. Who is that body? It is the gufa d’malka, the body of the Shechinah, the very being of the Queen. The ultimate hope of prayer, of dipping into the mikvah and of death is to be drawn back into that body and then be poured out yet again into an ongoing recycling of Divine and human soul. Thus, as the body dips itself into the mikvah, the soul dips itself into the higher spheres, up to the level of the River of Light.
Sources:
Rabbi Schneuer Zalman of Liadi, adapted from "Ohr HaTorah"
Rabbi Zalman Schachter-Shalomi, adapted from "Spiritual Intimacy"