There are two major problems in the theory of hydrodynamics. One is its nonlinear equations. If a fluid motion is specified by giving its velocity at every point, one may inquire whether there is a second possible motion in which the fluid everywhere has just twice the previous velocity.
Eckart, Carl. 1955. "Statistical hydrodynamics." Journal of Marine Research 14, (4). https://elischolar.library.yale.edu/journal_of_marine_research/842