Here in Oklahoma, USA, we are are in an area called "Tornado Alley" that has more tornados than anywhere else on earth. According to The National Oceanic & Atmospheric Administration, "Tornado Alley" is a nickname given to an area in the southern plains of the central United States that consistently experiences a high frequency of tornadoes each year. This area runs from South Dakota, thru Nebraska, Kansas, & Oklahoma to central Texas.
Some of these tornados are so violent, the only way to survive them is to be underground when one hits your location. 20 miles south of my home, an F-5 tornado's wind speed reached 318 MPH, the highest ever recorded. In 2013, 8 miles to our west, the El Reno tornado was 2.6 miles wide, the largest one ever recorded.
What is the significance of this? To survive one as described, you must be below ground when it passes over you. To accomplish that, you must build an underground storm shelter, AKA cellar. Mine is right outside the front door under the porch roof overhang (per my wife's request), made of "single-poured" concrete. It is completely without cracks or leaks after 30 years.
The down side of it is enough moisture seeps through the concrete to cause mildew & a musty smell. I solved this problem by installing a moist air evacuation fan. I'll explain in the following steps.
Step 1: VIEWS OF THE STORM SHELTER
The first pic was taken from the front yard, hiding the cellar door.
In the second pic the trap door can be seen with front door in background.
The 3rd is view of trap door.
The 4th shows the door open showing the steps.
The 5th is looking down into the cellar with light off.
The 6th shows interior with standard size bed in foreground and the vent pipe back in the right rear corner.
Last one shows view of other end of bed. 110 vac light on wall above bed.
The storm shelter measures 8 feet wide by 10 feet front to back, and is 6.5 feet tall.
When I was a Firefighter we had a bed frame with 2 standard sized mattresses, one above the other, like bunk beds only bigger. We spent many nights sleeping down there during stormy weather.
Step 2: Supplies & Tools
1 - 110v 120mm x 120mm fan
Electric zip cord to power fan
Single wall vent pipe 5 inch diameter x 6 feet long
Sheet Metal to make box for fan
I used tin snips and screws to assemble square metal box for fan.
Step 3: THE VENTING SYSTEM
The underground shelter has 2 vents 5 inches in diameter, one in each of the back corners. The air enters the fan at floor level since moist humidity lies at the lowest level. It is expelled up and out through the vent pipe. Warmer dryer air enter the other vent from outside to replace the air I exhaust. The fan runs 24/7.
The cellar is also a great place to store home canned fruits & vegetables. See last photo of fruit jars on shelves.
I cannot show photos of me making it since it was made 30 years ago. About 2 years ago the fan motor quit, so I replaced it with one from eBay. Not much expense for 30 years and it works great. If you have a cellar, consider this idea. It's totally dry and has no musty smell that's usually associated with an underground shelter.