山盟海誓 (shānménghǎishì): to pledge undying love

Found in the anonymous Yuan-dynasty zaju 《碧桃花》(Bìtáohuā, Green Peach Blossom): 他将山盟海誓言 (he made vows by mountain and sea). The image — love as durable as the mountains' weight and the sea's vastness — became the standard romantic oath in Chinese literature.

memory

山 (mountain) + 盟 (pact) + 海 (sea) + 誓 (oath) — literally “mountain-pact, sea-oath.” Picture two lovers swearing on something as permanent as the landscape itself.