难以忘怀的意思：

形容记忆深刻，不容易忘掉。

英文解释

unforgettable;difficult to forget;

中英例句

因此，除了满足园中4000头哺乳动物和7000只鸟儿的需要，更大的挑战是确保每年的360万名游客能与大自然和野生动植物实现意义深远、难以忘怀的亲密接触。

Therefore, besides satisfying 4000 mammals and 7000 birds in our collection, our greater challenge is to ensure that the 3.6 million visitors each year have meaningful and memorable encounters with nature and wildlife.