  Given a sequence of asymptotically flat 3-manifolds of nonnegative scalar
curvature with outermost minimal boundary, converging in the pointed $C^0$
Cheeger--Gromov sense to an asymptotically flat limit space, we show that the
total mass of the limit is bounded above by the liminf of the total masses of
the sequence. In other words, total mass is lower semicontinuous under such
convergence. In order to prove this, we use Huisken's isoperimetric mass
concept, together with a modified weak mean curvature flow argument. We include
a brief discussion of Huisken's work before explaining our extension of that
work. The results are all specific to three dimensions.
