Simplify the following text for a Grade 9 native English-speaking person.
Original: Adits are usually built into the side of a hill or mountain , and often occur when coal or ore is located inside the mountain but above the local valley floor or coastal plain .
Simplified: Adits are driven into the side of a hill or mountain , and are often used when an ore body is located inside the mountain but above the adjacent valley floor or coastal plain .