Fleming's discovery was serendipitous! He had left a Petri dish uncovered, and mold spores landed on it. When he returned from vacation, he noticed the mold had killed the bacteria. He famously said, "One sometimes finds what one is not looking for." Penicillin was first used to treat a human infection in 1941. By 1944, it was being mass-produced for Allied soldiers. Fleming warned about antibiotic resistance: "The microbes are educated to resist penicillin." Today, antibiotic resistance is a major global health crisis. Fleming was a modest man; he was knighted in 1944. He died of a heart attack in 1955. The discovery of penicillin has been called the greatest medical breakthrough of the 20th century. The word "antibiotic" comes from Greek "anti" (against) and "bios" (life).

DNA is a double helix! Watson and Crick published a one-page paper in Nature in April 1953. Francis Crick is often quoted saying, "We have discovered the secret of life." The structure explained how DNA could replicate (unzip and copy itself). Rosalind Franklin's Photo 51, taken in 1952, was shown to Watson without her knowledge. She is now recognized as a key contributor. Watson and Crick both lived long lives; Crick died in 2004, Watson is still alive (born 1928). The Human Genome Project (completed in 2003) sequenced all 3 billion base pairs of human DNA. The discovery of DNA's structure is one of the most important scientific breakthroughs of the 20th century.

Columbus called the native people "Indians"! He thought Cuba was part of China and that the Caribbean islands were near Japan. He called the Taíno people "Indios" (Spanish for Indians). The name stuck for centuries. Columbus's voyages launched the Columbian Exchange: tomatoes, potatoes, maize, beans, squash, chili peppers, and tobacco traveled from the Americas to Europe; wheat, rice, horses, cattle, pigs, sheep, goats, and smallpox traveled from Europe to the Americas. Smallpox devastated native populations (up to 90% mortality). Columbus was not the first European to reach the Americas (Leif Erikson did around 1000 CE), but his voyages had lasting global impact.

Howard Carter discovered the tomb! He famously replied to Lord Carnarvon: "Yes, wonderful things!" when asked if he could see anything. The tomb was largely undisturbed because it had been buried under debris from later tomb construction. Tutankhamun became king at age 9 and died at age 19 (c. 1323 BCE). His tomb contained a solid gold coffin weighing 110 kg (240 lbs). The discovery was a global media sensation. The "curse" was invented to sell newspapers; statistically, expedition members lived normal lifespans. Carter spent ten years cataloging the artifacts. He died in 1939. Tutankhamun's artifacts are displayed at the Egyptian Museum in Cairo. A new Grand Egyptian Museum (near Giza) is planned to display the full collection.

The Rosetta Stone unlocked hieroglyphs! Before its discovery, Egyptian hieroglyphs were indecipherable. The stone dates from 196 BCE (Ptolemaic period). It records a decree issued by priests honoring Pharaoh Ptolemy V. The British seized the stone from the French after defeating them in Egypt (1801). The stone has been in the British Museum since 1802. Egypt has requested its return; the museum has refused. Champollion, a French linguist, cracked the code using the names of Ptolemy and Cleopatra. The term "Rosetta Stone" has become a metaphor for a key to solving a difficult problem. The stone weighs about 760 kg (1,680 lbs). It is one of the most visited objects in the British Museum.

The Dead Sea Scrolls are ancient Jewish manuscripts! They were first discovered by Bedouin shepherds. Over 900 scrolls were found in 11 caves. The scrolls include every book of the Hebrew Bible except Esther. They also include rules for religious communities (like the Essenes, who likely wrote the scrolls). The scrolls were written in Hebrew, Aramaic, and Greek. They were hidden to protect them from the Romans during the First Jewish Revolt (66-70 CE). The scrolls are stored at the Shrine of the Book in the Israel Museum, Jerusalem. The scrolls have been digitized and are available online. They are considered the greatest archaeological discovery of the 20th century.

Röntgen discovered X-rays! The discovery revolutionized medicine. Doctors could now see inside the body without surgery. X-rays were quickly adopted for diagnosing broken bones, finding swallowed objects, and treating cancer (radiation therapy). Röntgen also discovered the medical use of X-rays. He was a conscientious scientist who declined financial gain from his discovery. He donated his Nobel Prize money to the University of Würzburg. X-rays are a type of ionizing radiation; excessive exposure can cause cancer (leading to safety precautions). Röntgen's discovery also led to the discovery of radioactivity (Henri Becquerel) and radiation therapy. Röntgen died in 1923. The unit of radiation exposure (röntgen) is named after him.

Rutherford discovered the atomic nucleus! The gold foil experiment showed that atoms are mostly empty space. The nucleus is tiny (about 1/10,000 the diameter of the atom) but contains most of the atom's mass. Rutherford was a New Zealand-born physicist. He also discovered alpha and beta radiation and coined the terms. He is often called the "father of nuclear physics." His students included Niels Bohr (atomic model) and James Chadwick (discovered the neutron). Rutherford was a mentor to many future Nobel laureates. He was knighted and buried in Westminster Abbey (near Newton and Darwin). The element rutherfordium (Rf) is named after him.

Pangaea was the supercontinent! Wegener published "The Origin of Continents and Oceans" (1915). He was a meteorologist, not a geologist, which made geologists skeptical. Harry Hess (seafloor spreading) and others provided the mechanism in the 1960s. Plate tectonics explains earthquakes, volcanoes, and mountain formation (Himalayas, Andes). The continents continue to drift today – about 1-2 inches per year. The next supercontinent (Pangaea Ultima) is predicted to form in about 250 million years. Wegener's work is now celebrated as a major scientific breakthrough. The theory of plate tectonics is one of the most important discoveries in Earth sciences.

Jenner developed the smallpox vaccine! The word "vaccination" comes from "vacca" (cow). Jenner's work built on the observation that cowpox provided immunity to smallpox. He was a country doctor in Gloucestershire, England. He faced criticism from the medical establishment. Smallpox killed an estimated 300-500 million people in the 20th century alone. The WHO launched an eradication campaign in 1967. The last naturally occurring case was in Somalia in 1977. Eradication was declared in 1980. Smallpox vaccines are no longer routinely given. Samples remain in two labs (CDC in Atlanta, VECTOR in Russia). Jenner is buried in Berkeley, Gloucestershire. A statue of him stands in Trafalgar Square (London) and in Kensington Gardens. The Jenner Institute at Oxford University (developers of a COVID-19 vaccine) is named in his honor.