The Mysterious Mushroom (不思議なキノコ, Fushigi na Kinoko) is an event in Tomba!. A similar event can be found in the sequel, called Strange Fruit.
Find the mushroom that can switch between the status ailments laughing and crying.
Talk to the dwarf at the right side of center roof in Charity Square to activate the event. He will talk about a mushroom that can switch between the laughing and crying status ailment.
This mushroom can be found inside a 1,000 year old treasure chest high above the right side of the Mushroom Forest. The The 1,000 Year Old Man? event must be cleared in order to get the 1,000 Year Old Key.