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Episode 39: Chinese Invasion Alarm, 5 Second Rule

  • Pottery drums lowered in holes can detect an approaching tunnelling army: plausible
  • The Five Second Rule: mythbusted

Chinese Invasion Alaram

Myth: You can detect the tunnelling/mining of a beseiging army using pottery jars stationed below the ground.

The first started with an incorrect translation of the myth, which was that with a 80 liter pottery jars lowered into holes around the town, you can detect approaching armies. This is similar to the Hollywood portrayal of putting your ear to the ground to track your enemy. After getting the fact-checking sheets from the textbook publisher that included the myth, they were able to track it down to an ancient Chinese text by Mao Tzu written in 4th century BC.

Mao Tzu's text described a detection system for protecting walled cities for approaching tunnelling invaders. They system used 200 liter jars with 3 feet of water and covered with fresh goat skins. These drums were lowered into shafts 15 feet below the base of the city wall. The shafts were 5 paces apart and listeners would be stationed at each shaft. The intensity could be compared from different locations to determine where an approaching tunnelling army was.

During WWI, soldiers would listen in trenches and tunnels for the digging of opposing armies. When the enemy was detected they would blow up galleries full of explosive beneath the enemy position.

Grant and Kari prepared the drums. It turns out that Kari has some goat-shaving connections and got some proper advice on preparing the goat skin. Akbar the goat shaver recommended first using clippers to shave the skin, soaking it in water for half an hour, then stretching it. Finally, a disposable razor should be used to shave the remaining stubble.


They went to the Priest Mine, one of the oldest hard-rock mine in California (all hand-dug). They chose three locations in the mine: the first 50 ft away, the second below their shaft, and the third 50 ft deep and 100 ft away. Jamie and Tory were sent into the mine to do the pick-axing while Kari and Adam did the listening at the shaft. Grant manned a geophone and an audio frequency mic.

They tested their setup by picking away at the mine while Grant tried to detect the digging with the geophone and Kari listened at the drum-less hole. Grant was able to picked up the sounds of Jamie's digging but Kari couldn't hear anything.

The three tests

They lowered the drum in to start the tests. Kari listened with a traffic cone to focus the sounds and "Deaf as a post" Adam listened in as well.

At the first pick-axing location, Kari had no trouble picking up each strike with the pick-axe. She described it as a faint beating of the drum everytime the pick-axe hit and she believed that the shaft helped focus the sound. Grant also picked up faint signals with the geophone while "Deaf as a post" Adam heard nothing.

The simple drum in a hole seemed to equal or even outdo the performance of some of the more expensive that Grant had setup, including the audio frequency mic, which picked up nothing.

They moved to the second location beneath the hole, where Kari and Adam heard nothing. The earth was softer beneath the hole and it may also be possible that the detection system has a blind/deaf spot. Grant got faint signals on the geophone.

They moved to the final location 50 ft deep and 100 ft away. Kari was able to count off the strikes perfectly.

plausible: they were able to detect digging 50 ft deep and 100 ft away with the drum. They couldn't find any physical evidence of this ever being used, but the alarm worked so well and Mao Tzu's account was so detailed that it looks plausible.

The Five Second Rule

Myth: if you pick up a piece of food that has fallen on the floor before five seconds is up, no bacteria will get on it

See also: NYTimes article on Clemson University Five-Second Rule study, "The Five-Second Rule Explored, or How Dirty Is That Bologna?"

Test 1: MythBusters HQ samples

Jamie and Adam used contact plates to test various locations in the MythBusters shop for bacteria. Each contact plate was placed on the floor for five seconds and labelled with the location. They incubated the contact plates for 24 hours and then counted the number of bacteria on them.

They got different results from locations that were adjacent to each other, so they decided that it would be important to eliminate location as a variable in the test..

Mini-myth: Toilet seat is the cleanest spot in the house

While they were collecting samples around the shop, Adam also placed a contact plate on the toilet seat for comparison. It did have less bacteria colonies than the other samples, though it seems that the other samples were all collected from the floor.

Test 2: Evenly contaminated surface

In order to eliminate location as a variable they created some evenly contaminated surfaces with beef broth. They dropped wet food (pastrami) and dry food (cracker) onto the surface for two and six seconds. They also did a control sample for comparison.

The wet pastrami picked up more bacteria than the dry crackers, but there wasn't a discernable difference between the two-second and six-second samples. They would need to do more testing to single out time as a factor in the test.

Jamie: "I don't think the results were all that conclusive."

Mini-myth: Dog mouths are cleaner than human mouths

Adam and Lulu the Dog both licked a contact plate. After incubating the samples, the dog's sample had less bacteria colonies than Adam's.

Adam: "I don't eat my own poo"

According to LiveScience: "Despite a habit of licking things no human would dare, Fido's mouth is often touted as scientifically more sterile. Truth is, oral bacteria are so species-specific that one can't be considered cleaner than the other, just different."

Test 3: Contact plates on evenly contaminated surface

They decide to eliminate food as a variable in the test and instead just apply the contact plates to the beef brothed surface for two seconds and six seconds. Both showed tremendous amounts of bacteria with no real difference based on time.

They found that the amount of bacteria that was picked up depended on the moisture of food, the surface geometry of food, and the location that it was dropped on, but there was no correlation to the amount of time it was dropped.



I hate you guys, you made my 5 second real go bye bye.

hey you guy's i realy like ur show and watching this epasode today was realy amazing how tht worked like it did.

thnx a lot guy... u just finished my science fair project... well it started out from this too so thnx for the project!

you guys helped me out majorly with my review of literature for my science fair! thanks

In fact in WW1 british troops used biscuit tins on the floor of tunnels filled with water. However the way you worked it was that you put your ear "in" the water to hear better. Perhaps they put skins on top to stop rubbish falling into the water.

My family just finished a science fair experiment comparing five seconds versus ten seconds swabbing different surfaces and used petri dishes. We actually found that more bacteria grew in the ten second petri dishes than the five second petri dishes.

Heh ^^ thanks for the information guys :D

Really helped out with our science fair project :)

Hi there. I happened across your blog because I'm working with Gold Bug Park in Placerville (home of the Priest Mine) and noticed you
link to their website, which is appreciated. It's funny because I love Gold Bug Park, and my kids love Myth Busters, and I had no idea Myth Busters had gone to Gold Bug Park!

Anyway, I wanted to let you know the Gold Bug Park website has been updated, and was wondering if you'd mind updating your link? The page you linked to no longer exists, but the Priest Mine now has it's own page at I didn't see any other way to contact you, so am leaving this info in a comment.

Please let me know if you get a chance to update that, and I'll be sure to show my son your MythBusters Guides as well!


BTW, it looks like your old page is still getting indexed too?