  We demonstrate experimentally the existence of a purely elastic fingering
instability which arises when air penetrates into an elastomer confined in a
Hele-Shaw cell. Fingers appear sequentially and propagate within the bulk of
the material as soon as a critical strain, independent of the elastic modulus,
is exceeded. Their width depends non-linearly on the distance between the
confining glass plates. A key element in the driving force of the instability
is the adhesion of layers of gels to the plates, which results in a
considerable expense of elastic energy during the growth of the air bubble.
