The hollow was full of trees but the higher ground was covered with grass.凹地树木成林，但高处长满青草。

hollow, den, hole
参见hole条。

用作定语
▲～+n.
Bamboo is a sort of hollow plant.竹子是一种中空的植物。
He has hollow cheeks.他的双颊凹陷。
用作表语
▲S+be+～
A bar of iron is solid;a pipe is hollow.铁棍是实心的，而管子是空心的。
A drum is hollow.鼓是空心的。
The words you say are very hollow.你讲的话很空洞。
He declared that he was telling the truth, but his words rang hollow.他自称说的都是真话,但听起来不像真的。