The Oyu Tolgoi mine is a combined open pit and underground mining project in Mongolia. It is a joint venture between the Government of Mongolia and Turquoise Hill Resources, a majority owned subsidiary of Rio Tinto. At the public hearings held during the comment period for the DEIS, commenters expressed concern over potential schedule delays and technical complications with the block cave mining technique proposed for use in the underground operation at the Oyu Tolgoi mine.