a larger photometric sample). Furthermore, @widrow2012, @williams2013, @carlin2013, and @sun2015 observed bulk motions in Solar Neighbourhood disc stars perpendicular to the midplane (vertical motions). These motions can be interpreted as a combination of the bending and breathing modes that are theoretically predicted for a plane-symmetric system [@mathur1990; @weinberg1991; @gomez2013; @widrow2014; @widrow2015; @gomez2016; @gomez2017].

The existence of warps has been attributed to the tidal effects of satellite galaxies, interactions of the disc with its dark halo, internal excitation of bending instabilities in gaseous discs, and intergalactic winds and magnetic fields, to name just a few [@kahn1959; @hunter1969; @sparke1984; @sparke1988; @battaner1990; @binney1992; @debattista1999; @lopez2002; @revaz2004; @shen2006]. Satellites and halo substructure have been invoked to explain the bending and breathing waves seen in the Milky Way [@widrow2012; @gomez2013; @widrow2014; @feldmann2015; @d'onghia2016; @gomez2016; @gomez2017], though these features can also be generated by spiral structure [@debattista2014; @faure2014; @monari2016a], the bar [@monari2015], or some (non-linear) combination of the two [@monari2016b].

The conclusion is that a galactic disc continually experiences perturbations that can set up waves perpendicular to its midplane. The question remains as to what happens to these waves once they are produced. Are they long-lived or do they quickly decohere, heating and thickening the disc? 