The 42-year-old now owns his own 4ha jumbo-sized sandpit in Steamboat Springs, Colorado, sharing his collection of big boys' toys, which includes 10 tonne bulldozers and 15 tonne excavators with anyone willing to pay up to $1000 for a full day.
"I think subconsciously we never grow out of our sandboxes . . . when we see a big scraper we all think 'I would love to have a crack on that'."
The idea came to him when he was clearing his Colorado section with a hired excavator.
"I thought this is so much fun. Imagine all these people who have never had a chance to do this."
Buying ex-hire excavators, bulldozers and loaders, Mr Mumm opened his business, Dig This, last November.
"It was not a cheap business to set up," he said.
The gamble to open what is believed to be the only heavy machinery playground in the United States had paid off, and he is now investigating opening up other playgrounds there and abroad.
"I might even open up one in Auckland if there is an opportunity."
Mr Mumm grew up in Waldronville, then studied at the Telford Rural Polytechnic before becoming a general farm worker in Millers Flat and other farms around the South Island.
After helping run the Omarama rodeo, Mr Mumm travelled to the United States with some riders back in 1992.
"I wasn't planning on staying, but I love it here. I met my wife Carla, and that was that."
People from all walks of life had taken to his business venture, learning how to control the massive machines in classroom sessions before releasing their inner child by moving earth and digging trenches.
"People just love it. We are on to a winner," he said.
Planning to return to Dunedin to visit family and friends in next month, Mr Mumm hoped he would have more luck than on his previous visit.
In January last year, Mr Mumm was diving off Taieri Mouth when he got into difficulties and had to be helped to the surface and revived.
