  This work is concerned with the Biot/squirt (BISQ) models for wave
propagation in saturated porous media. We show that the models allow
exponentially exploding solutions, as time goes to infinity, when the
characteristic squirt-flow coefficient is negative or has a non-zero imaginary
part. We also show that the squirt-flow coefficient does have non-zero
imaginary parts for some experimental parameters. Because the models are
linear, the existence of such exploding solutions indicates instability of the
BISQ models. This result calls on a reconsideration of the widely used BISQ
theory. Furthermore, we demonstrate that the 3D isotropic BISQ model is stable
when the squirt-flow coefficient is positive. In particular, the original Biot
model is unconditionally stable where the squirt-flow coefficient is 1.
