  The propagation distance of a pulsed beam in free space is ultimately limited
by diffraction and space-time coupling. "Space-time" (ST) wave packets are
pulsed beams endowed with tight spatio-temporal spectral correlations that
render them propagation-invariant. Here we explore the limits of the
propagation distance for ST wave packets. Making use of a specially designed
phase plate inscribed by gray-scale lithography, we synthesize an ST light
sheet of width $\approx700$~$\mu$m and bandwidth $\sim20$~nm and confirm a
propagation distance of $\approx70$~m.
