硐 has little standalone use in standard Chinese. As a variant of 洞 it means a cave or mine gallery (common in mining and southwestern place names); read 硐 it is an old word for a stone grinding mill. The stone radical 石 marks the rock sense; 同 supplies the sound.
register
Dialectal and archaic. In standard Mandarin write 洞 for a cave; 硐 stays in mining usage and regional place names.
