Free and open to the public year-round, Cylburn Arboretum is Baltimore's largest public garden, encompassing 207 acres. Built in the 19th century as a private estate, the Arboretum is home to a historic mansion and carriage house, as well newer buildings created to welcome and educate visitors that were designed using green techniques, including composting toilets, a “living” green roof, and geothermal heating and cooling. The Arboretum is the site of numerous free programs, including weekly yoga classes, outdoor concerts, and nature-focused workshops. It also houses a teaching farm operated by the Johns Hopkins Center for a Livable Future that features aquaponics, a system of agriculture combining fish farming with hydroponic plant farming.