Hasan Basri (may Allah have mercy on him) says that it reached him that the Noble Prophet (peace and blessings be upon him) said: The wound caused by an animal is without liability, the blood of one who dies by falling into a well is without liability, the blood of one who dies in a mine is also without liability, and for buried treasure (found by someone), one-fifth (khums) is obligatory.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 10394