Black holes are undoubtedly one of the most bizarre and intriguing phenomena in the universe. We’ve only known about them for about a hundred years, though their existence has been postulated since the 18th century. What is a black hole, anyway? What really happens when you fall into one? Astronomers and physicists have been wrestling with black hole mysteries for years, but a growing consensus has emerged on a number of points.

On the whole, American cities have become much safer. After years of declining rates of violent crime, they are now about as safe as they were in the 1960s. But violent crime is still a problem in lots of cities. And while the gap between urban and suburban crime rates has narrowed, suburban spaces still tend to be safer than urban places. Echoing that trend, it seems the safest cities in America are predominantly suburbanized middle-tier cities with moderate population density.

Does déjà vu hint at parallel universes? Is it evidence of reincarnation from a past life? Or is it simply a curious mental error, a telltale glitch in our brain that hints at the architecture of the mind? It depends on who you ask. Déjà vu invokes a sudden sensation of familiarity, most commonly in response to a scene, but the stimulus could also be a person, place, or even just a passing thought.

Like a monster trash compactor stretching from the ground to the heavens, tornadoes descend with little warning from the planet’s scariest storms, taking just a few minutes to ruin thousands of lives. Like any natural disaster worth its salt, tornadoes remind us of how puny we humans and our built structures really are. Here are 12 cool facts about tornadoes to keep you from getting bored as you crouch in a dank, dark cellar hiding from the most violent storm on the planet.

Racism is like an invasive species. As a mindset and expression of larger cultural attitudes, it tries to overwhelm its competitors wherever it sets seed. Defining victory as someone else’s oppression, racism causes imbalance and conflict in societies in which multiple races or ethnicities of people live and work. It promotes hostility within a culture, and ultimately leads to hatred and violence. Why are people racist? Why can’t we all just get along?

Everyone forgets things some of the time. You walk into a room and entirely forget why you decided to enter. You are mid-conversation and suddenly space-out what was just said. Although odd and sometimes embarassing, short-term memory loss below a certain threshold doesn’t mean anything at all. However, if you are having significant, unexpected and unexplained trouble remembering things that just happened, it could be a symptom of a greater problem.

These days, it’s not uncommon to encounter a person with a gluten allergy or celiac disease. According to the Statistic Brain website, about 1 in every 133 people in the U.S. have celiac disease. Restaurants and food manufacturers are becoming more aware of this and adjusting their menus and product offerings accordingly. However, there are hidden sources of gluten in many foods. Also, some people may experience gluten allergy symptoms and not know that the reactions are related to diet.

Many types of scenery are beautiful, but few geological features are as dramatic, breathtaking, and awe-inspiring as mountains. Mountains soar high above our heads, astounding us with their heights and features. They make us feel tiny, vulnerable, and fragile, but they also lift us up at the same time, inspiring us to strive for new heights. Here are 10 of the most beautiful mountains in the world. Some of these peaks are famous, while others are not as well known.

Filling a sunken land at the lowest point on earth, the Dead Sea in the Middle East is one of the world’s most fascinating places. It’s an incredible salty lake–one of the world’s saltiest bodies of water, in fact. As a giant solution in which 45 billion tonsof salts has been dissolved, it’s about 10 times saltier than ocean water. This is not the kind of salt used to season meals.

History is full of mysteries, partly because the people that gave rise to them failed to account for the insatiable curiosity of future mystery-mongers by leaving adequate evidence behind. Fair enough, but that shouldn’t stop us from mystery-mongering and treasure-hunting to our hearts' content. For all the Robert Stack-inspired sleuths in the audience, here are your top 10 unsolved historical mysteries. 10. Phaistos Disc (c. 1700 B.C.) A second-millenium artifact covered in hieroglyphics no one can read, the Phaistos Disccontains a magical curse, a geometric proof, a militant call to arms — or perhaps the meaning of the cosmos as understood by the ancient Minoans.