Although ocean acidification has only emerged as a global environmental issue recently, it quickly raises researchers and societies’ concerns about its impact on oceans and marine life. This investigation explores how human activities cause ocean acidification and thus affect the marine life. A wide range of sources are used to construct this investigation: primary sources such as academic thesis and reports published by authoritative organisations including NOAA, NASA, and IEA; secondary sources including online articles and documents. Multimodal presentations such as documentaries and interviews are also employed to conduct this study. The investigation discovers that human activities are the main causes of ocean acidification. Research suggests that oceans absorb large portions of excess CO2 emitted via humans, and hence causes ocean acidification. Moreover, the investigation shows that ocean acidification is harming calcifying organisms, and it also holds potential threat to many other marine species. Therefore, the investigation suggests that humans need to take measures immediately to cut down the CO2 emission, so that ocean acidification can be stopped or at least decelerated.