NES Code Name Viper Stage End Bug & Secret place

At the end of any level hold down "Turbo Jump" or "Down" button(and "Fire" when you need). There is a secret underworld on the first level. You can see enemies from other levels. 1:42 secret place It happens when you get within 50 or so pixels at the end of the stage and your character immediately goes into the slow walking mode and throws that grenade to open door.