cavern; cave; cave system; grotto
cave; hollow; cavity; hollow
cave; cavern; den; grotto
insight; discernment; perception; perspicacity; acumen
sinus; cavity; antrum; neighborhood (administrative division in North Korea)
limestone cave; limestone cavern
making hollow; making meaningless; deindustrialization; deindustrialisation
service tunnel (cables, gas, etc.)
tree hollow; hollow in a tree
venous sinus; sinus venosus
dongxiao (Chinese bamboo flute similar to a shakuhachi)
sinus arrhythmia; irregular heartbeat
sinus node; sinoatrial node
(animal) whiskers; vibrissae
druse; globular calcium oxalate crystals, often lining a rock cavity
sand martin (Riparia riparia)
brown-throated martin (Riparia paludicola); brown-throated sand martin