Anchors two famous chengyu:
沧海一粟 (cānghǎi yī sù, 'a grain in the ocean' — a drop in the bucket) from Su Shi's 《
前赤壁赋》 (Red Cliff Rhapsody, 1082 CE), and
沧海桑田 (cānghǎi sāngtián, 'the blue sea becomes mulberry fields') describing world-altering change.
沧 (cāng) specifically means 'dark blue-green', the color of deep water.