Fragmentation of a nuclear system by tunneling is discussed in a molecular
dynamics simulation coupled with an imaginary time method. In this way we o
btain information on the fragmenting systems at low densities and temperatu
res. These conditions cannot be reached normally (i.e., above the barrier)
in nucleus-nucleus or nucleon-nucleus collisions. The price to pay is the s
mall probability of fragmentation by tunneling but we obtain observables wh
ich can be a clear signature of such phenomena.