LONDON - An artist is inviting Londoners to come face-to-face with the wretched labour of one of India's lowest castes - by filling an art gallery with 21 big blocks of human excrement. The monoliths are the brainchild of Santiago Sierra, whose previous work includes pumping a former German synagogue full of poisonous car exhaust (visitors wore gas masks) and an attempt to write the word "Submission" in giant, flaming letters near the U.S.-Mexico border. Elena Crippa, curator at the Lisson Gallery where the work is being displayed, said the Spanish-born artist's intention is to confront audiences with the horror...