I just learned of Shivabalayogi's tapas of meditating for 23 hours a day for eight years. And 12 hours a day for two more years. That is the most notable tapas I have heard of. What are some other notable tapas that modern yogis have done?
Sri Ramana Maharishi meditated in a cellar-like temple called Pathala Lingam , which is a part of the Arunachala Temple ( A board to this effect, can still be found there. I have visited the temple).
He shifted to an underground vault known as Patala Lingam, where the sunlight never penetrated. Without moving he sat deeply absorbed in the Self and was unaware of being bitten up by the ants and vermin living there.
he didn’t even realize when some devotees finally came, lifted him out of the pit and brought him to the nearby Subrahmanya shrine. For about two months he stayed in that shrine paying no attention to his bodily needs. To make him eat, food had to be forcefully put into his mouth.
Similar accounts of Sri Ramakrishna are found in books about his years of spiritual effort.
Swami Vivekananda's account of his meeting with one Pavahari Baba is quite famous and is mentioned in the various biography books on Swami Vivekananda. Pavahari Baba is said to have not come out of his cave for five years.