People are injecting saline into their foreheads to make the skin bubble. The process takes about two hours, and looks absolutely horrifying. To achieve the "bagel-head" look, they press a finger in the center of the bubble during the infusion. The look only lasts for one night.
The process was brought to Japan by journalist Ryoichi Maeda who has been documenting extreme body modification (apparently, it's a big underground scene) there since 1992.
Maeda learned about the process in 1999 at an extreme body modification convention (yes, they exist) and then assembled a team to start offering the infusions in Tokyo in 2007.
Apparently, the saline injections aren't even considered that extreme. Amputation and naval removal are also popular practices.