  Bound states in the continuum (BICs) exist in a variety of physical systems
where they appear as lossless propagating states surrounded by radiating modes.
However, in the case of open systems, they coexist with continuous families of
guided states, which may be modes or other BICs, located in different regions
of the frequency-momentum parameter space. Here we report that waveguiding
structures comprising anisotropic materials with two radiation channels where
continuous families of modes are not possible whatsoever can support discrete
BICs that exist for a single frequency and a single propagation direction. The
results are isolated spectrally-discrete BIC states existing as lossless
needles emerging from a sea of radiating leaky modes.
