What is it?
A resurgent dome is a hill formed by subterranean magma movement.

Really? Do, tell.
Akin to a lava dome, resurgent domes are dynamic, growing and shrinking over time. A resurgent dome occurs where there is an underground magma chamber. When volcanic activity increases in the area, more magma flows through and about the chamber, causing the dome's size to increase. When the activity subsides, the dome is reduced.

What is this activity like?
Well, the magma chambers in question are usually far beneath the surface (often as many as six or seven kilometers), so we can only speculate as to what going on there. However, increasing resurgent dome size is often accompanied by an increase in the frequency and magnitude of earthquakes in the region.

Now, since each resurgent dome is unique, it moves to its own epochal rhythm. Some change size only very slowly, with alterations that are measurable over many decades. Others are far more nimble, changing in size as much as ten centimeters in an average year. This differs in times of volcanic upheaval, when all bets are off. If an eruption occurs nearby, a dome may suffer a significant reduction in short order, at the loss of pressure. Conversely, a resurgent dome that seems to be straining at the seams would mean that you should probably leave the engine running in case a quick getaway is required.

Where might I find such a thing?

Please, say something else.
The earth breathes.