1) The phenomenon known as "black holes" occurs when matter is crushed into an infinitely small space, creating a gravitational pull so strong that nothing can escape.

2) The Appalachian Mountains, running along the eastern coast of the United States, are one of the oldest mountain ranges in the world.

3) The ozone layer, located in the Earth's stratosphere, protects us from harmful ultraviolet (UV) radiation.

4) The French Revolution, which began in 1789, marked a major turning point in European history, leading to the overthrow of the monarchy and the rise of republicanism.