In one of the dev docs for bo1 multiplayer they tinker with the idea of having a post launch story. Specifically there are two map descriptions that detail the Soviets attempt to weaponize an ancient supervirus "Apophus" that until recently was frozen in the ice of Antarctica.

Behold. The very first call of duty emblem, created by Treyarch employees from WaW assets when pitching the idea for Black Ops 1.

[Black Ops] [Marketing] Did you know Black Ops 1 had an ARG during its marketing season called GKNOVA6? Very similar to Cold War's PawnTakesPawn ARG it was used to market campaign and zombies! As you'll probably notice this is where I got my PFP from. More info below!

[Aether] [Narrative Development] The Der Riese clock is the conceptual root of the cycle in the zombies storyline. Always ticking, resetting itself, and repeating all over again. Ad Infinitum. Source can be found below!

[Dark Aether] [Official Canon] Dr. Oskar Strauss has a very long, storied career. Born on May 19th, 1918 in Germany. After finishing university in 1943, Strauss was recruited by the Uranverein. He was promised an opportunity to explore a new frontier of atomic research with Projekt Endstation; however, wanting to improve humanity rather than destroy it, he promptly requested transfer once he found out Endstation's mission statement. Unfortunately, this only led to him being transferred to a much nastier Projekt. One headed by Dr. Friedrich Steiner working on Nova 6. During his time working for the Third Reich, Strauss personally knew scientists who were developing exotic weapons such as the Haunebu II Flying Saucer, and even the infamous Die Glocke; a multiversal version of which can be seen in Totenreich in Black Ops 7 Zombies! After the end of World War II, Strauss defected to the United States during Operation Paperclip. He would then find work under DARPA before eventually joining Requiem in 1983. The rest is history! Sources can be found below!

[Aether] [Official Canon] The reason the Earth's destruction severely weakens Ultimis Richtofen's control over the Aether is due to it causing a massive rift in space-time. You may be familiar with whenever Richtofen speaks of "mending the Rift", or whenever Victis directly point out the Rift as the massive hole in the mantle at the start of Buried. What exactly the Rift is can be gleaned in quotes throughout Black Ops 2 zombies, and later a radio in Tag Der Toten which gives even more context. Essentially all objects both animate and inanimate contain some trace amount of 115 (the innate energy produced by the Aether). The denser the object, the more 115 there within. Whenever Maxis destroys the Earth in bo1, such enormous amounts of Aetheric energy spilled out that it damaged the fabric of reality. It is this very same energy that you channel through the Towers in bo2 for either Richtofen or Maxis. At the end of Buried, Maxis successfully harnesses this energy via manipulating Victis across each map in order to reach Agartha. Maxis effectively does the opposite of mending the rift, instead using it to COMPLETELY destroy the Earth after the events of Buried. Sources can be found below.