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.