Science with the ngVLA: Extreme Scattering Events and Symmetric
  Achromatic Variations
  Radio variability in quasars has been seen on timescales ranging from days to
years due to both intrinsic and propagation induced effects. Although
separating the two is not always straight-forward, observations of singular
`events' in radio light curves have led to two compelling, and thus far
unresolved mysteries in propagation induced variability--- extreme scattering
events (ESE) that are a result of plasma lensing by AU-scale ionized structures
in the interstellar medium, and symmetric achromatic variability (SAV) that is
likely caused by gravitational lensing by $\gtrsim 10^3\,M_\odot$ objects.
Nearly all theoretical explanations describing these putative lenses have
remarkable astrophysical implications. In this chapter we introduce these
phenomena, state the unanswered questions and discuss avenues to answer them
with a $\sim $weekly-cadence flux-monitoring survey of $10^3-10^4$
flat-spectrum radio quasars with the ngVLA.
