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I think the answer would be The Rocky Mountains.
Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by groundwater circulating through them. As the rock dissolves, spaces and caverns develop underground.
Sinkholes are formed when the land surface above collapses or sinks into the cavities or when surface material is carried downward into the voids. Drought, along with resulting high groundwater withdrawals, can make conditions favorable for sinkholes to form.
Slowly, deep underground
The igneous rock gabbro is coarse grained and non-vesicular. Both characteristics show that it is an intrusive igneous rock. Intrusive igneous rocks are formed below Earth's surface and slowly (because they're underground). When rocks are intrusive, they are are non-vesicular (no air pockets) because the air has had the time to escape because of the slow cooling time period. Intrusive rocks are also coarse because crystals had the time to form in the rock during that longer cooling time.
Ps. extrusive igneous rocks form rapidly at Earth's surface and basically have tge opposite characteristics for the opposite reasons (ex: vesicular cause air had little time to escape)
It is Important because an unusual occurrence in the realm of international relation