  For low-dimensional chaotic attractors there is usually a single number of
unstable dimensions for all of its periodic orbits and we can say such
attractors exhibit "mono-chaos". In high-dimensional chaotic attractors,
trajectories are prone to travel through quite different regions of phase
space, some far more unstable than others. This heterogeneity makes
predictability even more difficult than in low-dimensional homogeneous chaotic
attractors. A chaotic attractor is "multi-chaotic" if every point of the
attractor is arbitrarily close to periodic points with different numbers of
unstable dimensions. We believe that most physical systems possessing a
high-dimensional attractor are of this type. We make three conjectures about
multi-chaos which we explore using three two-dimensional paradigmatic examples
of multi-chaotic attractors. They can be thought of as small-scale examples
that give insight for real high-dimensional phenomena. We find a single route
from mono-chaos to multi-chaos if an attractor changes continuously as a
parameter is varied. This multi-chaos bifurcation (MCB) is a periodic orbit
bifurcation; one branch of periodic orbits is created with a number of unstable
dimensions that is different from the mono-chaos.
