When people hear the word "mushroom", they call to mind a very small part of a fungus. Toadstools (or mushrooms) are the reproductive fruiting body that contains spores -- the fungi's equivalent of seeds. The fruiting body, whose job is to develop and release spores, develops quickly and lasts only a few days or weeks. Toadstools mostly pop up above the surface of soil, on rotting wood or on some other other dead item that the fungus relies upon as a food source.

But fungi are much larger organisms than the short-lived "toadstool" that we typically see. Most of the fungus is concealed from view underground. The concealed portion of fungi consists of white mycelia and black rhizomorphs that live on and dead or dying wood and tree roots. I still recall my surprise and wonder upon learning that the largest organism on Earth is not the blue whale, as I had presumed, but actually is a colony of fungi residing underground in the Malheur National Forest, located in eastern Oregon state. Molecular studies revealed that this one individual fungus is huge; estimated occupy 2,200 acres (8.9 km2) and is probably more than 2,400 years old.

But different fungi species can be found nearly everywhere in the world. For example, did you know there are more than 14,500 species of mushrooms in the UK alone? And plenty of these fungi live in London and other towns and cities, as we learn in a video from mycologist Mark Spencer, who is an expert on fungi at London's Natural History Museum.

In this delightful video, we accompany Dr Spencer as he creeps through London's underbrush to share glimpses of "micro-forests of fungi" with us, and explains their vital ecological contributions -- even though they are found in a large city.

Although the cool and damp autumn is the best time to seek out fungi, some species produce fruiting bodies at other times of the year, too. Wildlife and birds eat fungi and especially their fruiting bodies. Red squirrels, Sciurus vulgaris, are particularly found of mushrooms, which they also gather and store in their winter larders.

If you are planning to eat mushrooms (my favourite preparation is to sauté a wide variety of mushroom species in fresh herbs and white wine sauce), it is important to know that some fungi -- even those living in urban areas -- are extremely poisonous.

As Dr Spencer cautions us: "If you are thinking about eating fungi, you need to be absolutely sure they are safe."

"Make sure you get expert advice, and join a local group of forayers to build up your experience."