In the Junkyard, right before entering the room to fight Noiman Cascade, if the player climbs up the wall in there all the way to the top and off the screen, they will arrive to a secret room where they will meet the Mysterious Character. He will ask them if they want to fight in the Battle Arena; if the player agrees, they will be taken to a crowded fighting arena where they will fight three bosses in succession: Simondo Belmont, Psycho Mother and Huge Face.
Huge Face appears to be a mixture of flesh and metal with a fiendish face at the center and steel plates on its sides. It can launch bombs from its mouth, which will detonate after a set period of time (displayed on a counter on the bombs themselves). It can also launch homing missiles.
It can escape into an inter-dimensional portal and emerge in another one in a different location as well, although this is known to be very unstable and potentially cause a fold in the time-space continuum, leading to time traveling results.