溶洞

guì lín yǒu hěn duō róng dòng

桂林有很多溶洞。

There are karst caves in Guilin.

yóu yú shuǐ de bù duàn qīn shí rì jiǔ tiān cháng jiù xíng chéng le yī gè dà róng dòng

由于水的不断侵蚀，日久天长就形成了一个大溶洞。

As a result of continuous water erosion, a huge cave was formed after a long time.