I liked the variety of guesses for this one, from "miners digging out the last of our coal and/or gold reserves" to "it has something to do with a dam being built at the bottom of Hahamongna."
Frank Girardot wins with his 3:56 p.m. Tuesday guess "building of the rose bowl in arroyo seco."
I tried to throw everybody off the scent by cropping in on this photo:

Architect Myron Hunt designed the Rose Bowl Stadium, which for several years was a horseshoe shape that was open on the south side. Here's Hunt, right, looking over an early model with builder William A. Taylor:

And here's an aerial shot of the nearly completed stadium:

But the project really began with laborers and mules. Imagine the back-breaking work of digging out all that dirt along with all those rocks and boulders.

In 1920, to fund the stadium's $272,000 construction, Myron Hunt, Tournament of Roses President William Leishman and several others devised a plan to sell subscription tickets for stadium events for the next 10 years at a cost of $10 per person per year -- $100 total -- all up front. It worked.
Construction took less than a year; the stadium was ready in time for the 1923 football game with USC vs. Penn State (USC won, 14 to 3).

And now the stadium is poised for a $171 million renovation and expansion. Design plans are going through the commission process and the financial plan will be coming to the Pasadena City Council soon.
It's a far cry from the $272,000 raised to build the stadium 90 years ago but it sure will be worth it!
I'll leave you with this aerial shot of the stadium during the BCS final game on Jan. 7 this year (Alabama 37, Texas 21):
Many thanks to the Pasadena Museum of History, Rose Bowl Stadium and Tournament of Roses Association.









