  Only in the Milky Way is it possible to conduct an experiment which uses
stellar streams to detect low-mass dark matter subhaloes. In smooth and static
host potentials, tidal tails of disrupting satellites appear highly symmetric.
However, perturbations from dark subhaloes, as well as from GMCs and the Milky
Way bar, can induce density fluctuations that destroy this symmetry. Motivated
by the recent release of unprecedentedly deep and wide imaging data around the
Pal~5 stellar stream, we develop a new probabilistic, adaptive and
non-parametric technique which allows us to bring the cluster's tidal tails
into clear focus. Strikingly, we uncover a stream whose density exhibits
visible changes on a variety of angular scales. We detect significant bumps and
dips, both narrow and broad: two peaks on either side of the progenitor, each
only a fraction of a degree across, and two gaps, $\sim2^{\circ}$ and
$\sim9^{\circ}$ wide, the latter accompanied by a gargantuan lump of debris.
This largest density feature results in a pronounced inter-tail asymmetry which
cannot be made consistent with an unperturbed stream according to a suite of
simulations we have produced. We conjecture that the sharp peaks around Pal 5
are epicyclic overdensities, while the two dips are consistent with impacts by
subhaloes. Assuming an age of 3.4 Gyr for Pal 5, these two gaps would
correspond to the characteristic size of gaps created by subhaloes in the mass
range of $10^6-10^7 M_\odot$ and $10^7-10^8 M_\odot$ respectively. In addition
to dark substructure, we find that the bar of the Milky Way can plausibly
produce the asymmetric density seen in Pal 5 and that GMCs could cause the
smaller gap.
