An amphitheatre is a large open area surrounded by rows of seats sloping upwards. Amphitheatres were built mainly in Greek and Roman times for the performance of plays.
N-COUNT
（部分或整个被包围的）露天场地，圆形凹地
You can describe land which partly or completely surrounds an open area as an amphitheatre.
例句
...a natural amphitheatre of mountains.被群山环绕的天然露天场地