Bhagavad Gita 17.19मूढग्राहेणात्मनो यत्पीडया क्रियते तपः।परस्योत्सादनार्थं वा तत्तामसमुदाहृतम् ॥

Some burn sulphur in a pot and place it on their head. Some thrust hooks of iron into their flesh. Some hang themselves with their head downwards over fire and swallow smoke. Some stand in cold water immersed up to the neck. Some torture the body by lighting fires on the four sides (with the sun as the fifth fire -- this is known as the Panchagni Tapas). Some sit in the centre of a circle of fire. Such austerities are Tamasic. These will not help one to attain knowledge of the Self.