What is the Livyasan (Leviathan)?

In Bereishis 1:21, the Torah describes the creation of the taninim gedolim (big sea-giants). Rashi cites a Gemara in BB 74b that says this refers to the Livyasin and its mate that Hashem created, but killed the female and preserved it for tzadikim in the world to come, because if they would reproduce, the world wouldn’t be able to continue with them (because they’d take over the world). We analyze this gemara and suggest that the Livyasan refers to an amazing feature of Hashem’s Creation.