1736: Manhattan Project
Title text: On the plus side, we definitely killed that cancer over there, even if we caused a bunch more everywhere else.
The Manhattan Project was a big, expensive, secret research and development project that produced the first nuclear weapons during World War II. Because of the unprecedented scale of the project, which involved some of the brightest minds in science and the efforts of thousands of people, "Manhattan Project" has become a metaphor for any kind of all-out effort involving the top minds of a discipline to achieve a single objective, often expressed as the phrase "We need a new Manhattan Project       ".
The day before this comic was released the following announcement was made: Microsoft will ‘solve’ cancer within the next 10 years by treating it like a computer virus. And on the day this comic was released (but probably after the comic was released) there was a press conference where Mark Zuckerberg and Priscilla Chan Pledge $3 Billion to Fighting Disease (all disease in general.) This is the kind of projects that could be called Manhattan type projects and these (at least the Microsoft announcement) could be the reason this comic came out now.
In this comic, Ponytail starts by making the suggestion that they should create a "new Manhattan Project" to cure cancer. Taken figuratively, this would imply a heavily-funded, massive collaborative effort involving the best scientists in the field of cancer research, and is not an unreasonable idea in itself. However, she and her fellow scientists all take the idea literally instead, and the New Manhattan Project ends up actually developing a nuclear bomb. In the final panel, Ponytail appears to realize that this runs somewhat counter to her original objective  (not to mention is redundant, as the original Manhattan Project already invented the nuclear bomb).
The title text hastily justifies this mistake by claiming a partial success; that their nuclear detonation did, indeed, kill all cancer within the blast radius of the explosion. However, it fails to mention that the blast would also kill everything else as well. It also admits that the explosion would most likely end up causing more cancer due to the ionizing radiation and fallout. The title text is reminiscent of both the main comic and the title text of 1217: Cells.
This was the first time since 1355: Airplane Message, more than two years ago, that Randall mentions cancer (on a banner!), a recurring subject on xkcd, but mainly around the time when his then-fiancée (now wife) was diagnosed with Stage III breast cancer in October of 2010. Interestingly enough there are only two facts in that previous comic, and that other fact (from the title text) was referenced the week before this comic came out in 1732: Earth Temperature Timeline. But it could be a coincidence.
The Manhattan Project was the scene of 809: Los Alamos, and a story from the site is being told in 1520: Degree-Off. According to 980: Money the Manhattan project used $24,400,000,000. Nuclear weapons in general has been a recurrent subject on xkcd and their invention was also mentioned last week in 1732: Earth Temperature Timeline around 1950 CE. The previous comic with a similar mushroom cloud was 1655: Doomsday Clock, and in that comics explanation at least three other "recent" comics about such weapons of mass destruction are mentioned.
- [A five panel layout with each panel slightly smaller than the next.]
- [Ponytail holding her arms out speaks to Cueball.]
- Ponytail: What we need to cure cancer is a new Manhattan Project
- [Cueball stands next to Ponytail on podium with a banner overhead. Ponytail is lifting her arms high up and addresses a huge crowd below the podium. Faces disappear into the distance, but at the podiums edge are four full faces, from left to right they are Hairy, a person with flat hair, a person with white hair and a Cueball-like guy.]
- Banner: Research Initiative
- [Cueball and Megan sitting behind a desk looking out and Ponytail standing to the right facing away from them wear laboratory goggles and laboratory coats. There are several Erlenmeyer flasks on the desk and Ponytail is also holding such a flask. There are other glass wares on the desk.]
- [Megan, holding a hand in front of her face, Cueball and Ponytail, all wearing some kind of glasses strapped around the back of their heads stand behind two chest-height barriers looking into the distance where a large mushroom cloud rises high in the air with the typical ring around the stem below the main cloud and smoke/dust surrounding the bottom of the stem. It is much higher in the image than the three mountains in the left background.]
- [Close up of Ponytail, as she faces to the right. She is wearing very dark protection glasses, looking like those used for looking at the sun during a solar eclipse.]
- Ponytail: Wait.
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