There is a 'roll top lid' that covers the opening. After shoving your sleeping bag inside the sack, you place the lid over the opening. The lid has 4 straps that are connected to the bottom of the sack. Once the lid is in place, pull the 2 opposite straps at the same time. The lid is made out of the same waterproof fabric as the rest of the sack, sometimes it is even double insulated because it has more stress put on it. As long as you make sure the lid covers the entire opening, no water will be able to get in.
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There is a 'roll top lid' that covers the opening. After shoving your sleeping bag inside the sack, you place the lid over the opening. The lid has 4 straps that are connected to the bottom of the sack. Once the lid is in place, pull the 2 opposite straps at the same time. The lid is made out of the same waterproof fabric as the rest of the sack, sometimes it is even double insulated because it has more stress put on it. As long as you make sure the lid covers the entire opening, no water will be able to get in.