The hike to Thunder Lake starts at the Wild Basin trailhead in Rocky Mountain National Park, it's a common trailhead to get to other destinations within the Wild Basin Area, as such it's a crowded trail up until the split for Ouzel Lake. After that the crowds thin out dramatically and you pretty much have the trail to yourself.
For me, this hike came in just over 15 miles roundtrip. As I have been on the first leg of this trail a couple of times in the past, the first part went pretty fast. Additionally, you are in a dense canopy of tall Pines so it's a bit dark and cool.
You don't see Thunder Lake until you are right at the shore line, once there it is breathtaking. It looked like a postcard of the Colorado Rockies. The isn't a lot of beach-front property along the shore of Thunder Lake so you must find little openings to get to the shoreline. It is very peaceful there. I wish I was able to stay there overnight.
It was partly cloudy for part of the time I was at Thunder Lake, making for some dramatic shadows on the granite walls of the bowl that Thunder Lake sits in. When the clouds cleared out, it was simply amazing. What a beautiful lake. Worth going back to as there are many other lakes nearby.