Timer slowly running out. Once the circle completely fills, the bomb explodes.
Heavy Bombs appear during the encounter with Sabetha the Saboteur. While players fight one of her lackeys (Kernan, "Knuckles", or Karde), Sabetha retreats to her bomb piles and occasionally tosses one onto the platform. They need to be kicked before they explode or they will deal heavy damage to the platform. The time left before a heavy bomb explodes is indicated by the orange circle above a bomb, which slowly fills the longer a bomb is up.