If you'd like to read a little more about the original humongous fungus there's a really interesting article by Tom Volk, first published in Inoculum in 2002, a decade after the discovery of the fungus. You can read the article online here. The fungus they discovered, an individual Armillaria bulbosa, aka the honey mushroom, was conservatively estimated to be at least 1500 years old and weigh around 100 tons - making it one of the largest and oldest living organisms.
I learned lots of fascinating trivia from this article:
The project was actually an offshoot of a grant from the Department of Defense, which funded a project to study the possible biological effects of ELF (Extra Low Frequency) stations in the Upper Peninsula of Michigan. These ELF stations were built to communicate underground with ocean-going submarines in time of war.
They sampled for the fungus by 'baiting' with tongue depressors. The fungal mycelium quickly colonized the wood sticks.
They were not looking for a large fungus, or even trying to measure the size of any fungus. The project was originally to look at how mitochondrial DNA was inherited in fungi in nature.
When news of the 'giant fungus' broke in the press CNN wanted someone to go out into the woods and wave from the fungus so they could get an aerial picture of the humongous fungus.
Even better, a Japanese businessman called and wanted to build a boardwalk around the humongous fungus and charge people to view the 'pulsating mass of fungus'.