Even when the bombs don't penetrate the concrete, the shock waves are disabling to the inhabitants. Since the bunker is in a fixed position, once the artillery is zeroed in on it, you just pound it till it breaks, and there's nothing the inhabitants can do about it. Ground attack airplanes firing at the gun ports, enveloping the bunker with napalm, phosphor bombs, etc., all are effective and fairly cheap to do.

For accounts of this by survivors,

"D Day Through German Eyes" by Eckhertz

Also, already in WW II the "normal" bomb shelters (also mentioned in the article) weren't effective protection to the methods proudly used by the British and US war planners:

https://emedicine.medscape.com/article/833665-clinical

"Increased ambient environmental temperature from burning napalm has been known to cause the deaths of individuals in raid shelters as a result of radiant heat and dehydration. This was a frequent cause of death in the bombing raids carried out over Hamburg, Germany, during World War II."

Recently Swedish forest fires swept across a military range. Because of fear of unexploded ordinance, the fire fighters couldn't pursue it.

But the air force dropped a very small conventional bomb, sucked all the oxygen out of the air, and put out the fire.

It was in the news a bit. A quick google finds https://www.thelocal.se/20180725/watch-swedish-fighter-jets-...

Didn't they drop the idea again, because it didn't really work that well?

My point about camouflaging it by having a nice turf over it was to avoid its position being fixed in the first place.

What is the point? either you construct it in secret and hope the secret is safe (no spys!), or you construct it in public so the enemy already knows where it is. In the first place you hope the enemy doesn't know. In the second place you know the enemy already knows so you mitigate that by either having so many that the enemy cannot deal with them all, and/or you mitigate by making the strong enough to withstand an attack.

These didn't have anything rational about them. They are literally everywhere- seemingly randomly placed in the middle of farm fields, etc. and they kept people busy and were a response to the ever present "threat" of the US and/or USSRs ever imminent attack. So camouflage didn't matter.

> It would be such a little thing to dump the earth they excavated when building the bunker back on top of the thing and spread it around so grass grew etc, hiding the damn thing.

Easier said than done, and I don't think camouflaging them was ever a priority. Even today, you can see the excavated material spilling down the hill in front of large bunkers 70+ years later, which makes them fairly easy to locate: https://www.cios.org.je/assets/images/other/noirmont-headlan... (this location is also easy to spot in wartime RAF reconnaissance images that I've seen)