  We report on the direct measurement in real space of the effect of the van
der Waals forces between individual Rydberg atoms on their external degrees of
freedom. Clusters of Rydberg atoms with inter-particle distances of around 5
{\mu}m are created by first generating a small number of seed excitations in a
magneto-optical trap, followed by off-resonant excitation that leads to a chain
of facilitated excitation events. After a variable expansion time the Rydberg
atoms are field ionized, and from the arrival time distributions the size of
the Rydberg cluster after expansion is calculated. Our experimental results
agree well with a numerical simulation of the van der Waals explosion.
