In the process of researching her book Bonk: The Curious Coupling of Science and Sex, author Mary Roach found herself unable to answer one simple question: "What actually happens during intercourse?" While we always assumed it was pretty obvious, it turns out scientists have long struggled to come up with an empirical explanation, mainly because they can't actually see what the hell is going on. So when Roach heard that a researcher in London was looking for a couple to lend their privates to the first-ever "four-dimensional ultrasound" sex movie, she grabbed her mortified husband and volunteered. Then she posted the footage on Slate, which you can check out above. Prepare to be alternately mesmerized and grossed out.