Basically you drive up Coal Creek Canyon (out by Rocky Flats off 93) until it hits the Peak to Peak highway. Turn left. Drive to where the rail line comes up to the Moffat Tunnel. Turn right on a gravel road which - if you took it to the end - ends at the Moffat Tunnel. Shortly before the end, there is a right, another gravel road, with switchbacks leading up over Rollins Pass.
The road over the pass is still closed by the landslide/rockfall (AFAIK), but its cool to drive up to the point where its closed, esp. if the weather is nice.
Yankee Doodle Lake is fairly small, and toward the top. Opposite the lake is a cave, you can walk around inside it. Think its an old mining site. I never made the dive, but after walking the ground the only thing it could be would be a railcar type device going into the mine. (No other access for larger technology that I could see...) It would be an interesting dive on a bright sunny day, definitely a high altitude dive. Bring drysuits!
If you find anything, take pictures!
Best,
Doc