  Quantum theory dictates that upon weakening the two-body interaction in a
three-body system, an infinite number of three-body bound states of a huge
spatial extent emerge just before these three-body states become unbound. Three
helium atoms have been predicted to form a molecular system that manifests this
peculiarity under natural conditions without artificial tuning of the
attraction between particles by an external field. Here we report experimental
observation of this long predicted but experimentally elusive Efimov state of
$^{4}$He$_{3}$ by means of Coulomb explosion imaging. We show spatial images of
an Efimov state, confirming the predicted size and a typical structure where
two atoms are close to each other while the third is far away.
