人山人海 (rén shān rén hǎi)

(idiom) multitude; vast crowd

Character breakdown

Usage

国庆节的天安门广场人山人海。

Tiananmen Square was packed with people on National Day.

Guóqìngjié de Tiān'ānmén Guǎngchǎng rénshānrénhǎi.