Given a scientific passage and an answer, generate a question for the given answer.

Passage: Sometimes an aquifer is confined. A confined aquifer is trapped between two impermeable rock layers. Pressure from the rock layer on top forces the water out where the aquifer reaches the ground surface. Water that flows up to the surface naturally is an artesian spring . If people drill a well into a confined aquifer, the water may flow to the surface without assistance. This is an artesian well ( Figure below ). Answer: confined
What type of aquifer is trapped between two impermeable rock layers?