Coba jelasin Uga delengen dari Gua Optymistychna dalam bahasa jawa dong

Description of a 1990 trip to Optymistychna by British cavers

The Fund of Support Scientific and Creative Initiatives

Chapter II.9, Gypsum Karst in the Western Ukraine by Alexander Klimchouk article appearing in: International Journal of Speleology, vol. 25, #3-4, 1996, p. 263-278.