Known as the Oracle of Oil, Oklahoma native T. Boone Pickens built a vast fortune through his success in the petroleum industry. In the 21st century, Pickens' career took an unexpected turn as he emerged as a vocal advocate for renewable energy and conservation. Deeply committed to environmental stewardship, he used his Mesa Vista ranch as a sandbox for his beliefs about considerate land management.

Beyond his environmental efforts, Pickens also left a significant philanthropic legacy. While he lost his billionaire status in the years before his death, he ultimately donated more than $1 billion (£759m) to charitable causes during his lifetime, according to TopTenRealEstateDeals.com.

Inside oil tycoon T. Boone Pickens’ $170m Texas ranch