Fun Fact: The "Drill Curls" haircut is actually a real hairstyle that has been around for a very long time. I'm talking hundreds, possibly thousands, of years old. However, they aren't normally called "Drill Curls", but instead go by the name "Ringlets."

They've also been called "Corkscrew Curls" in the past, but the anime version that people love is commonly known as the Regal Ringlet. The anime versions are just exaggerated versions of the Regal Ringlets where the top of the hair is more pronounced than the bottom.

Source: Wikipedia & Jandelaine, The Aesthetician.