Underground Blowout
An underground blowout is the flow of formation fluids from one zone to another. Most commonly, we can recognize the underground Blowout by a lack of pressure response on the annulus. And this response will be while pumping on the drill pipe or by a general lack of pressure response. The underground Blowout can be most difficult, dangerous, and destructive. It can be most difficult because the conditions are not clear and can evade analysis.
https://www.drillingmanual.com/underground-blowout/