Literature Reference: 沙虚留虎迹，水滑带龙涎。—— 《晚投慈恩寺呈俊上人》 Tiger prints remain on the sandy void, while dragon's drool glides on the slippery water. Source from 'Wan Tou Ci'en Si Cheng Jun Shang Ren'.
沙: Sand, grains of sand.
虚: Illusion. Idleness. Vanity.
Meaning: A tranquil and profound scene, symbolizing peace and depth.