Anchors two famous chengyu:
沧海一粟 ('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 ('the blue sea becomes mulberry fields') describing world-altering change.
沧 specifically means 'dark blue-green', the color of deep water.