In the 1940s and 1950s, underground shelters gave citizens the peace of mind that, in the event of a nuclear catastrophe, they'd be able to survive. Those shelters are still used in areas prone to tornadoes and other natural disasters. Today, people still attempt to prepare for man-made or natural disasters by building underground shelters, which range from crude pits to high-tech concrete bunkers. Inexpensive yet effective shelters fall somewhere in-between.
Dig a hole in the ground. Using a shovel, dig a 12-foot-wide, 16-foot-long and 8-or-more-foot-deep hole. Line the hole with the plastic sheeting and tape the seams with the waterproof tape to prevent moisture from leaking in.
Build the frame. Using the 2-inch by 6-inch boards, build an 11-foot-10-1/2-inch by 7-foot-10-1/2-inch by 8-foot frame for the underground shelter. Be sure to frame up an entry point large enough to get people, supplies and equipment inside.
Cover the frame. Using the plywood, cover the frame and secure each panel with the deck screws. Seal each seam with silicone caulk. Build a hatch to cover the entrance to the shelter. Install the vent tubes so the shelter can get fresh air.
Carefully cover the shelter with the plastic wrap. Take care not to puncture the plastic so the shelter is waterproof. Fill the hole in around the shelter with the dirt removed while digging the hole. Cover the shelter with dirt. Make sure that the entrance and the vent tubes don't get covered.