  As a refinement of the Swampland Distance Conjecture, we propose that a
quantum gravitational theory in an infinite distance limit of its moduli space
either decompactifies, or reduces to an asymptotically tensionless, weakly
coupled string theory. We support our claim by classifying, as special cases,
the behaviour of M-Theory and Type IIA string theory compactifications on
Calabi-Yau three-folds at infinite distances in Kahler moduli space. The
analysis comprises three parts: We first classify the possible infinite
distance limits in the classical Kahler moduli space of a Calabi-Yau
three-fold. Each such limit at finite volume is characterized by a universal
fibration structure, for which the generic fiber shrinking in the limit is
either an elliptic curve, a K3 surface, or an Abelian surface. In the second
part we focus on M-Theory and investigate the nature of the towers of
asymptotically massless states that arise from branes wrapped on the shrinking
fibers. Depending on which of the three classes of fibrations are considered,
we obtain decompactification to F-Theory, or a theory with a unique
asymptotically tensionless, weakly coupled heterotic or Type II string,
respectively. The latter probes a dual D-manifold which is in general
non-geometric. In addition to the intrinsic string excitations, towers of
states from M2-branes along non-contractible curves become light and correspond
to further wrapping and winding modes of the tensionless heterotic or Type II
string. In the third part of the analysis, we consider Type IIA string theory
on Calabi-Yau three-folds and show that quantum effects obstruct taking finite
volume infinite distance limits in the Kahler moduli space. The only possible
infinite distance limit which is not a decompactification limit involves
K3-fibrations with string scale fiber volume and gives rise to an emergent
tensionless heterotic string.
