Back in the early 90s, scientists from the U.S. National Library of Medicine came up with a bold idea: imaging the entire human body, from top to bottom, in cross-sectional segments only a millimeter thick. They called the endeavor The Visible Human Project.
Two cadavers, one male and one female, were encased in gelatin and then frozen into ice blocks, at frigid temperature of -160 degrees Fahrenheit (-71 degrees Celsius).
Then, a sophisticated cryogenic microtome (basically a very finely-tuned deli slicer in a big freezer) was used to slice the cadavers, millimeter by millimeter. For the man, that meant a total of 1,878 slices (putting him at about 6′ 2″).
After each pass of the slicer, the remaining block of ice/body was photographed using high-resolution magnetic resonance imagery (MRI).
The following GIF is basically a 10 second slide show of those images:
The video below goes into more depth about the Visible Human Project, and also includes more internal body reconstructions like the one above.
After the GIF above was posted to reddit, another reddit user (u/thealphamike) decided to add some more detail to it.
Using the Adobe software Phototoshop and After Effects, he created the more graphic (and slightly creepy) GIF below:
(h/t IFL Science)