10 most difficult dis­cov­er­ies riddles

Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. (NGC 2264 shown; see also the Horsehead Nebula)

A fast radio burst (FRB) is a high-energy astrophysical phenomenon manifested as a transient radio pulse lasting a few milliseconds on average. The origin of the FRBs has yet to be determined. Proposals range from a rapidly rotating neutron star and a black hole to an extraterrestrial intelligence.

The mulberry is a significant fiber crop in the history of paper. It was used for papermaking in China by around 100 AD. The paper recipe used by Cai Lun included plant bark fibres, rags and fishing nets. Early paper makers particularly valued Mulberry for producing high quality paper. Washi, a Japanese handcrafted paper, is made with the inner bark, which is pounded and mixed with water to produce a paste, which is dried into sheets.

The original stereoscopic apparatus was invented in 1838 by Sir Charles Wheatstone from Gloucestershire, in the year of the birth of photography, yet his early experiments employed drawings instead of photographs, viewed through a system of angled mirrors.

The name America was first recorded in 1507. Christie's auction house says a two-dimensional globe created by Martin Waldseemüller was the earliest recorded use of the term. America derives from Americus, the Latin version of Italian explorer Amerigo Vespucci's first name.

John Snow was an English physician and a leader in the adoption of anaesthesia and medical hygiene. He is considered one of the fathers of modern epidemiology, in part because of his work in tracing the source of a cholera outbreak in Soho, London, in 1854. His findings inspired fundamental changes in the water and waste systems of London, which led to similar changes in other cities, and a significant improvement in general public health around the world.

Captain Matthew Flinders RN was an English navigator and cartographer, who was the leader of the first circumnavigation of Australia and identified it as a continent. Flinders made three voyages to the southern ocean between 1791 and 1810. In the second voyage, George Bass and Flinders confirmed that Van Diemen's Land (now Tasmania) was an island. In the third voyage, Flinders circumnavigated the mainland of what was to be called Australia, accompanied by Aboriginal man Bungaree.

Danish physicist and chemist Hans Christian Oersted (1777-1851) discovers in 1820 that electric current affects a compass needle and create magnetic fields. He was the first scientist to find the connection between electricity and magnetism. He is remembered today for Oersted's Law.

DNA was first isolated by the Swiss physician Friedrich Miescher who, in 1869, discovered a microscopic substance in the pus of discarded surgical bandages. As it resided in the nuclei of cells, he called it "nuclein". In 1878, Albrecht Kossel isolated the non-protein component of "nuclein", nucleic acid, and later isolated its five primary nucleobases.

Armalcolite is a titanium-rich mineral with the chemical formula (Mg,Fe2+)Ti2O5. It was first found at Tranquility Base on the Moon in 1969 and is named for Armstrong, Aldrin and Collins, the three Apollo 11 astronauts. Together with tranquillityite and pyroxferroite, it is one of three new minerals that were discovered on the Moon.