This sugya contains the famous aggadot about Leviathan, including its immense size and the cosmic role of such creatures in divine creation and future redemption.
רַב יְהוּדָה הִינְדְּוָא מִשְׁתַּעֵי: זִימְנָא חֲדָא הֲוָה אָזְלִינַן בִּסְפִינְתָּא, וַחֲזֵינַן הָהוּא אֶבֶן טָבָא דַּהֲוָה הָדַיר לַהּ תַּנִּינָא. נָחֵית בַּר אָמוֹרָאֵי לְאֵתוּיַהּ, אֲתָא תַּנִּינָא קָא בָּעֵי לְמִבְלַע לַהּ לִסְפִינְתָּא. אֲתָא פִּישְׁקַנְצָא פַּסְקֵיהּ לְרֵישֵׁיהּ, אִתְהֲפִיכוּ מַיָּא וַהֲווֹ דְּמָא. אֲתָא תַּנִּינָא חַבְרֵיהּ, שַׁקְלַהּ וְתַלְיַהּ לֵיהּ וַחֲיָה. הֲדַר אֲתָא קָא בָּעֵי בָּלְעָא לִסְפִינְתָּא, הֲדַר אֲתָא צִיפְּרָא פַּסְקֵיהּ לְרֵישֵׁיהּ. שַׁקְלוּהָ לְהַהִיא אֶבֶן טָבָא שַׁדְיוּהָ לִסְפִינְתָּא; הֲוָה הָנֵי צִיפְּרֵי מְלִיחִי בַּהֲדַן, אוֹתְבִינְהוּ עֲלַיְיהוּ; שַׁקְלוּהָ וּפְרַחוּ לְהוּ בַּהֲדַהּ. (סִימָן: כׇּל, שָׁעָה, יַרְדֵּן) אָמַר רַב: יְהוּדָה אָמַר רַב: כֹּל מַה שֶּׁבָּרָא הַקָּדוֹשׁ בָּרוּךְ הוּא בְּעוֹלָמוֹ – זָכָר וּנְקֵבָה בְּרָאָם. אַף לִוְיָתָן נָחָשׁ בָּרִיחַ וְלִוְיָתָן נָחָשׁ עֲקַלָּתוֹן – זָכָר וּנְקֵבָה בְּרָאָם, וְאִלְמָלֵי נִזְקָקִין זֶה לָזֶה – מַחְרִיבִין כָּל הָעוֹלָם כּוּלּוֹ. מָה עָשָׂה הַקָּדוֹשׁ בָּרוּךְ הוּא? סֵירַס אֶת הַזָּכָר, וְהָרַג הַנְּקֵבָה וּמְלָחָהּ לַצַּדִּיקִים לֶעָתִיד לָבֹא, שֶׁנֶּאֱמַר: ״וְהָרַג אֶת הַתַּנִּין אֲשֶׁר בַּיָּם״.
Rav Yehuda from India relates: Once we were traveling in a ship and we saw a certain precious stone that was encircled by a snake. A diver descended to bring it up, and the snake came and sought to swallow the ship. A raven came and cut off its head, and the water turned into blood due to the enormousness of the snake. Another snake came, took the precious stone, and hung it on the dead snake, and it recovered. It returned and again sought to swallow the ship, and yet again a bird came and cut off its head, took that precious stone, and threw it onto the ship. We had with us these salted birds; we placed the stone on them, and they took the stone and flew away with it. § The Gemara provides a mnemonic for the following statements of Rav Yehuda citing Rav: Everything; time; Jordan. Rav Yehuda says that Rav says: Everything that the Holy One, Blessed be He, created in His world, He created male and female. Even leviathan the slant serpent and leviathan the tortuous serpent He created male and female. And if they would have coupled and produced offspring, they would have destroyed the entire world. What did the Holy One, Blessed be He, do? He castrated the male and killed the female, and salted the female to preserve it for the banquet for the righteous in the future. As it is stated: “And He will slay the serpent that is in the sea” (Isaiah 27:1).