  Photonic bound states in the continuum are spatially localised modes with
infinitely long lifetimes that exist within a radiation continuum at discrete
energy levels. These states have been explored in various systems where their
emergence is either guaranteed by crystallographic symmetries or due to
topological protection. Their appearance at desired energy levels is, however,
usually accompanied by non-BIC resonances, from which they cannot be
disentangled. Here, we propose a new generic mechanism to realize bound states
in the continuum that exist by first principles free of other resonances and
are robust upon parameter tuning. The mechanism is based on the fundamental
band in double-net metamaterials, which provides vanishing homogenized
electromagnetic fields. We predict two new types of bound states in the
continuum: i) generic modes confined to the metamaterial bulk, mimicking
electronic acoustic waves in a hydrodynamic double plasma, and ii) topological
surface bound states in the continuum.
