We discovered a natural amphitheatre formed by volcanic activity during our hike.

amphitheatre 由前缀 amphi-（表示"环绕、两边"）和 theatre（剧场）组成，字面意思是"环绕式剧场"。这个词特指古罗马时期建造的圆形或椭圆形露天剧场，现代也用来形容类似形状的自然地貌。