Having an underground bunker is like the holy grail for survivalists. Not only are they virtually indestructible, the cost alone is often too much for the average person. There's nowhere safer to store your supplies or to hide out a disaster.
If it's been built right.
The trouble is, most people don’t have thousands and thousands of dollars to drop on a professionally built bunker. Plus, there's the whole OPSEC concern, as everyone who comes to work on your property now knows about the bunker. So, they choose build their own. Which comes with a variety of different dangers, that need to be planned for.
Running out of air
Adequate ventilation is key for an underground bunker. Without clean air you've just created a coffin for you and your family. Setup a system that pulls in fresh air from the outside world and filters out any contaminants, but make sure that the vents don't give away the position of your bunker. These need to be both camouflaged, and have redundancies in case one particular vent is discovered or compromised.
Filling up with water
This is a challenge from two angles. First, you need to ensure that any rainwater is not filtering into your bunker and filling it up. Second, you need to ensure that you've not dug down to the level of your local water table for your bunker. If it's below, I can guarantee you that it will flood, and every preventative measure you take will ultimately fail. Water will always win, no matter if it's rusting through your steel foundations or seeping through the concrete.
Collapsing under the earth
There's more to building safe underground structures than dropping a shipping container into a hole and covering it with dirt. You need an experienced contractor to help design the plans, otherwise the weight of the soil (not to mention any impacts like an earthquake or blast) will simply crush whatever structure you've been working from. I can't stress enough the danger of trying to survive a crisis in a natural cave system or an old mine shaft, and I'd never get into an underground shipping container. You simply don't know how safe it is.
Catching on fire
If you're going to spend any amount of time in your bunker it's logical you'll need fire to cook and heat with. Plus, many insulating materials are flammable, along with most of the supplies you've brought in. And you never know what may spark a fire. It could be a short in one of the electrical systems, or it may be a stray cigarette. Ensure you've taken steps to reduce the amount of "flammable" material in your bunker, and that you've got the right extinguishers handy too. If a fire does start you want it out as fast as possible.
The spread of mould
When you have a group of people in a confined space, every breath adds humidity to the air. Couple this with a wet environment, and suddenly there's a problem with moisture control. Which leads to mould. For some people this can cause a flare up of allergies, skin irritation, and even lead to respiratory illness chronic lung disease. Plus, who wants to be breathing mould spores all day. Ensure you've got a plan to dehumidify the air, and adequately vent areas like the kitchen, laundry and the bathroom.
Getting stuck underground
The trouble with do-it-yourself jobs is that these are often done too simply. Having a single entrance to your bunker is a recipe for disaster, especially if there's a problem that blocks your exit. Any one of a thousand possibilities could block a single exit, from a branch falling to attackers sitting in wait for you and your family to emerge. If your exit is blocked, you will die. Create a plan for your bunker that not only has a main entry point, but has a separate escape tunnel just in case. Oh and you can't go wrong with a few self-defense weapons, like a StrikePen or a Stinger. You'll thank me one day. Trust me.
Disposing of your waste
Getting rid of your waste is a challenge when you're camping, but as soon as your underground the problem multiplies a thousand-fold. You need a system that can adequately dispose of both human waste, along with the paper, plastic, food scraps and more that you'll create living underground. Plus, you need to consider hygiene, as rotting meat and waste creates gases that are harmful to breathe, and is a breeding ground for nasty bacteria that could be fatal to you and your family.
Hiding in plain sight
People are without a doubt your biggest concern after a SHTF event, and everyone who knows where your bunker is presents a threat. The contractors you used and the neighbors who saw it being craned in, will all remember the "hidden" cache of supplies you've now got on your property. And when things get bad, they will come looking. This is a danger that is very difficult to overcome, as you will need to use professionals to build a solid bunker. My only advice here is to hire from out of state, and to tie in any major construction work with additional property developments, like putting up a barn or installing a swimming pool, so you can perhaps get away with the bunker without raising a lot of suspicion.
Done right, a survival bunker is one of the best investments you can make. The key is to ensure you've got plans to overcome all of these dangers, and you're working hard on creating a safe and secure space for your family to ride out a crisis. That's what survival is all about.