It is narrated from Sayyiduna Amr bin Awf bin Milhah al-Muzani (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "(Damage caused by falling into) a well carries no liability; the injury caused by an animal (unintentionally) also carries no liability; (damage occurring while working in) a mine also carries no liability; and on buried treasure a fifth (zakah) is due."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14078