  As an effective theory, relativistic hydrodynamics is fixed by symmetries up
to a set of transport coefficients. A lot of effort has been devoted to
explicit calculations of these coefficients. Here we propose a shift in
perspective: we deploy bootstrap techniques to rule out theories that are
inconsistent with microscopic causality. What remains is a universal convex
geometry in the space of transport coefficients, which we call the hydrohedron.
The landscape of all consistent theories necessarily lie inside or on the edges
of the hydrohedron. We analytically construct cross-sections of the hydrohedron
corresponding to bounds on transport coefficients that appear in sound and
diffusion modes for theories without stochastic fluctuations.
