The first direct visual evidence of a black hole

On April 10, 2019, in coordinated press conferences across the globe, the Event Horizon Telescope (ETH) collaboration unveiled the image of a supermassive black hole at the heart of Messier 87 (M87), an elliptical galaxy 5.5 million light years away from us. This first direct visual evidence shows us a spherical black hole "shadow" and a crescent ring-line structure around the shadow, consistent with the prediction of Einstein's theory of general relativity. The EHT observations verify general relativity from the point of view of the strong gravitational field, and also help us to study the black hole's accretion and jets. This article will focus on why, which, and how black holes should be imaged, as well as describe the data reduction and data analysis. We further review the contributions of the Chinese astronomers in this collaborative project, and the prospects for future high-resolution imaging of black holes.