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Radio variability in quasars has been seen on timescales ranging from days toyears due to both intrinsic and propagation induced effects. Althoughseparating the two is not always straight-forward, observations of singular`events' in radio light curves have led to two compelling, and thus farunresolved mysteries in propagation induced variability--- extreme scatteringevents (ESE) that are a result of plasma lensing by AU-scale ionized structuresin the interstellar medium, and symmetric achromatic variability (SAV) that islikely caused by gravitational lensing by $\gtrsim 10^3\,M_\odot$ objects.Nearly all theoretical explanations describing these putative lenses haveremarkable astrophysical implications. In this chapter we introduce thesephenomena, state the unanswered questions and discuss avenues to answer themwith a $\sim $weekly-cadence flux-monitoring survey of $10^3-10^4$flat-spectrum radio quasars with the ngVLA.

Science with the ngVLA: Extreme Scattering Events and Symmetric Achromatic Variations