  A sound wave travelling in water is scattered by a periodic assembly of air
bubbles. The local structure matters even in the low frequency regime. If the
bubbles are arranged in a face-centered cubic (fcc) lattice, a total band gap
opens near the Minnaert resonance frequency. If they are arranged in the
diamond structure, which one obtains by simply adding a second bubble to the
unit cell, one finds an additional branch with a negative slope (optical
branch). For a single specific frequency, the medium behaves as if its
refractive index (relative to water) is exactly $n=-1$. We show that a slab of
this material can be used to design a three three-dimensional flat lens. We
also report super-resolution focusing in the near field of the slab and
illustrate its potential for imaging in three dimensions.
