After 12 years as a band (even though the oldest member is 25), several solo projects, and the stunning success of their Sugar Hill debut, it’s a shock to realize that This Side is only the second Nickel Creek album. I am stunned, however, that these kids having fun on bluegrass instruments have created a modern pop album demonstrably deeper and, in some ways, more sophisticated than Sgt. Pepper or Pet Sounds.

Produced by Alison Krauss, who herself uses bluegrass as the foundation for her own distinctive style, This Side makes a hard-to-refute statement: namely, that Nickel Creek is the strongest musical ensemble working today. As wonderful as they’ve proven to be, the young trio continues to exhibit extraordinary growth, even to those folks who are able to enjoy them live fairly regularly. Although Chris Thile’s opening instrumental, “Smoothie Song,” visits familiar Nickel Creek territory, “Spit on a Stranger” shows their advances in blending voices and instruments, in drawing from jazz and rock, in being Nickel Creek.

Repeated listening simply reveals greater depth. One has to search through John Coltrane and Miles Davis to find records as rewarding and fully realized as This Side. Yet these artists are in their early 20s, with a long way to go before reaching their full potential.