Find out what is inside the big ball of mud.
This event is activated directly after the previous event. Tomba and Zippo will hear noise coming from the second floor in Tabby's house. When they walk upstairs, the find a big lump of dirt. Seconds later, Gran shows up and offers himself to carry it down to the Underground Machine Room.
If you have cleared the event Bury it in the Sand!, Gran will go there directly, but if you haven't, he will tell you about it and force you to complete that event in order to progress in the game's story.
After that has been done, go down to the Underground Machine Room and talk to Gran who is standing next to the Washing Machine to complete the event.