Final answer:
Marlow uses the 'Great Stink' of 1858, a period when the River Thames was heavily polluted and emitted a terrible stench, to illustrate its once dark state, affected by industrialization and poor sanitary conditions.
Step-by-step explanation:
Marlow points to the example of the 'Great Stink' to characterize the Thames river as having once been one of the dark places of the earth. The term 'Great Stink' refers to the event in 1858 when an unusually hot summer exacerbated the levels of pollution in the Thames, which was heavily contaminated with human waste and industrial effluent due to the inputs from the revamped London sewer system. This pollution was so severe that it caused a terrible stench and even disrupted the operations of the British Parliament. Renowned scientist Michael Faraday's commentary reflects on the appalling condition of the Thames during that time, an indication of how industrialization and poor sanitation practices had profoundly degraded the river's ecosystem, affecting the health and wellbeing of London's inhabitants.