Besides enemy combatants, the tunnels themselves presented many potential dangers to tunnel rats. Sometimes they were poorly constructed and they would simply collapse. Tunnels were often booby trapped with hand grenades, anti-personnel mines, and punji sticks. The VC would even use venomous snakes (placed as living booby traps). Rats, spiders, scorpions, and ants also posed threats to tunnel rats. Bats also roosted in the tunnels, although they were generally more of a nuisance than a threat. Tunnel construction occasionally included anti-intruder features such as U-bends that could be flooded quickly to trap and drown the tunnel rat. Sometimes poison gases were used. A tunnel rat might therefore choose to enter the tunnels wearing a gas mask (donning one within was frequently impossible in such a confined space). According to U.S. tunnel rat veterans, however, most tunnel rats usually went without gas masks because wearing one made it even harder to see, hear, and breathe in the narrow dark passages.