Caving
Caving is a popular activity among student in Jogja. Several tour operators offer caving packages to visitors, including to the challenging earth-holes at the south limestone hills Pegunungan Seribu (Thousand Hills).
Cerme Cave at Imogiri is a popular one, for it being easily waded through. More challenging caves are Luweng Jaran, Grubug and Jati, with more than a hundred meters deep vertical tunnels.