LONDON—Artist Santiago Sierra is using human feces to bring audiences face to face with the lives of scavengers, or "untouchables," one of India's lowest castes, which has traditionally cleaned private toilets and outhouses in the country, the Chicago Sun-Times reports. Sierra's exhibition fills London's Lisson Gallery with 21 large blocks of excrement, gathered from Delhi and Jaipur in India, left to sit for three years, then mixed with plastic and molded into blocks. Sierra's past works include filling a German synagogue with poisonous car exhaust and sending visitors in wearing gas masks, and trying to write the word "Submission" in flaming letters near the U.S.-Mexico border.