This paper introduces the exponential substitution calculus (ESC), a new
presentation of cut elimination for IMELL, based on proof terms and building on
the idea that exponentials can be seen as explicit substitutions. The idea in
itself is not new, but here it is pushed to a new level, inspired by Accattoli
and Kesner's linear substitution calculus (LSC). One of the key properties of
the LSC is that it naturally models the sub-term property of abstract machines,
that is the key ingredient for the study of reasonable time cost models for the
$\lambda$-calculus. The new ESC is then used to design a cut elimination
strategy with the sub-term property, providing the first polynomial cost model
for cut elimination with unconstrained exponentials. For the ESC, we also prove
untyped confluence and typed strong normalization, showing that it is an
alternative to proof nets for an advanced study of cut elimination.