The Connections categories of the seven-level words are as follows, in increasing order of difficulty:

Countries
Armenia, Denmark, Ecuador, Vietnam.

Chemical elements
arsenic, calcium, krypton, mercury.

Technically fruit
apricot, avocado, coconut, pumpkin.

Anagrams
camelid, claimed, decimal, medical.

Plausible cryptic crossword anagram indicators
diced up, ordered, sketchy, various.

Objects counted with -枚 in Japanese
acrylic, judo mat, seaweed, SIM card

1 across: Arsenic might be an essential trace nutrient

Arsenic is famously toxic, and was historically favoured as an assassination tool since it was hard to detect and mimicked the symptoms of cholera. But studies in rats, goats, and birds have demonstrated arsenic deficiency is possible when fed unnaturally low levels of the element. (Arsenic naturally occurs in groundwater at levels of a few parts per billion.)

17 across: Microsoft slang references an email system from the late 80s

Microsoft employees use “OOF” as shorthand for “out of office”, even though it doesn’t make sense as an acronym. Reportedly, it comes from the name of the auto-reply feature in a Xenix email system Microsoft hasn’t used since 1993.

25 across: Yuri Oganesson

Yuri Oganessian is a physicist whose discoveries were essential to the discovery of elements 106 to 118, and who led the international team in Dubna, Russia who first synthesized many of them.

The race for the periodic table is a fascinating story in its own right; the team at Lawrence Berkeley National Laboratory claimed to have first produced elements 116 and 118 in 1999, but the discovery was later exposed to have been based on data fabricated by Victor Ninov. Both elements were later synthesized for real in a collaboration between the Lawrence Livermore National Laboratory in the US and Oganessian’s Joint Institute for Nuclear Research in Russia.

Oganesson is the heaviest element synthesized to this date; only a handful of atoms have ever been produced. It sits at the bottom of the noble gases column on the periodic table, but it is theorized that it would actually be a reactive solid if it existed long enough for those to be meaningful descriptions.

39 across: Oxygen is sour stuff

The word “oxygen” is named by Antoine Lavoisier after the Greek word ὀξύς describing sharp or harsh tastes, since he believed (incorrectly) that it was a component of all acids. In German, the element is called Sauerstoff.

49 across: Ada Lovelace was Lord Byron’s daughter

Ada Lovelace is famous in computer science as the author of the first published computer algorithm (for Charles Babbage’s Analytical Engine) and the first person to recognize that the machine could have applications beyond calculation:

Again, it might act upon other things besides number, were objects found whose mutual fundamental relations could be expressed by those of the abstract science of operations, and which should be also susceptible of adaptations to the action of the operating notation and mechanism of the engine. Supposing, for instance, that the fundamental relations of pitched sounds in the science of harmony and of musical composition were susceptible of such expression and adaptations, the engine might compose elaborate and scientific pieces of music of any degree of complexity or extent.

Ada Augusta, Countess of Lovelace

Less well known is Ada’s relationship to Lord Byron, the eccentric poet and famous philanderer. Lady Byron believed her husband to be insane and left Lord Byron shortly after giving birth to Ada (the marriage lasted one year). She strongly encouraged Ada’s scientific and mathematical studies in the hopes that she wouldn’t take after her father.

52 across: Osu!!

Osu (押忍) is an informal acknowledgement mainly used by people involved in martial arts. It’s also associated with cheer squads, and is referenced in the Japanese title of the Nintendo DS rhythm game series known in the west as Elite Beat Agents. It’s believed to be an extreme contraction of oyahō gozaimasu (おはようございます), or “good morning”, with the kanji 押 (“push”) and 忍 (“endure”) having been assigned after-the-fact.

54 across: Falling coconuts don’t kill that many people

There is an urban legend that hundreds of people are killed by falling coconuts each year. That statistic is completely unsubstantiated, but there is a seed of truth to it: a 1984 paper by Dr Peter Barss reported nine cases of serious head injuries caused by falling coconuts in Papua New Guinea.

68 across: Europe’s spaceport is in South America

Here’s a great trivia question: with what country does France have the longest border with? Is it Spain? Germany? No — it’s Brazil. That’s because of French Guiana, an overseas department of France between Brazil and Suriname on the north coast of South America. It’s part of the EU and Eurozone but not the Schengen Area.

French Guiana’s location right next to the equator made it the perfect site for the Guiana Space Center, built in 1968 after Algeria won its independence from France. The spaceport is operated by the European Space Agency (ESA) and the French and EU space agencies. The space sector is a significant fraction of the Guianese economy.

69 across: The giant stones of Yap

Yap is an island in Micronesia famous for its inhabitants’ use of large stone disks as a medium of exchange for ceremonial gifts. Rai, as the stones are known to the northern Yapese, have their owners recorded in oral histories as they are impractical to physically transfer.

They are valuable in part because there’s no limestone on Yap; the stones were quarried on Palau and transported 400km by boat. Supply of rai greatly increased after European contact, as did the disks’ individual sizes, but production stopped with the arrival of the Japanese in 1914 and many were lost to typhoon and World War II.

1 down: Avocados probably did not coevolve with giant ground sloths

A paper from the 1980s suggested that avocados might have co-evolved with giant ground sloths, who were large enough to eat and scatter the large seeds. But more recent research has that’s probably not true: avocados were smaller before human cultivation, and giant ground sloths didn’t live anywhere near there anyways.

5 down: Ice is technically a mineral

A mineral is a substance that is

• naturally occuring
• inorganic
• homogenous
• solid
• has a definite chemical composition, and
• has a crystalline structure.

Glacier ice checks all those boxes.

In response, Hank Green hyperbolically exclaimed on TikTok: “Ice is a rock, water is lava, and you are a lava monster. I guess??“

24 down: Tatami mats are interesting mathematically

Ten years ago I wrote a post summarizing some of the math behind traditional tatami patterns, so go check that out.

47 down: The United States has a lot of biomes

Back in high school, I participated in the Great Canadian Geography Challenge, and this crossword clue is my favourite question from provincials that I still remember two decades later.

You can find a lot of different biomes in the United States: tundra and taiga in Alaska; tropical rainforests in Hawaii; and coniferous forests, broadleaf forests, desert, prairies, flooded grasslands, mangroves, subtropical grasslands, and shrublands in the contiguous states.

48 down: The Three Sisters

Corn, beans, and squash (including pumpkins) are the three sisters of North America. This nutritionally-complete combination of crops is key to the cuisine of indigenous peoples across the continent. They featured prominently in the myths and diet of the nations of the Haudenosaunee Confederacy, for example.

59 down: Legendre’s constant

In 1808 Adrien-Marie Legendre conjectured that the prime-counting function π(x) asymptotically behaves like

for some constant $B \approx 1.08366$. Decades later, it was proved that the conjecture was right but the constant was wrong — in fact, B is equal to exactly one.

Hilariously, the only known contemporaneous image of Legendre is a random watercolour caricature by Julien-Léopold Boilly.

61 down: The seconds pendulum was almost the definition of a meter

The meter was originally defined as one ten-millionth of distance from the North Pole to the Equator through Paris. The main competing proposal was to use the length of a pendulum with a period of two seconds.

By sheer coincidence, the two numbers are almost exactly the same — the seconds pendulum has a length of about 0.993 meters (plus or minus less than a centimeter, depending on where you are on the Earth).