Chinese Idiom -- 人山人海

人(rén)山(shān)人(rén)海(hǎi) = people mountain people sea, a metaphor of many people 

Example: 
国(guó)庆(qìng)黄(huáng)金(jīn)周(zhōu)，中(zhōng)国(guó)到(dào)处(chù)都(dōu)人(rén)山(shān)人(rén)海(hǎi)。

During the golden week of National Day holiday, there are many people everywhere.