A vanishing and mysterious phenomenon, concentrated on the western alluvial slopes of the Sierra, spans much of California's Great Valley. Vernal pools. We most often hear about vernal pools in relation to the "endangered" fairy shrimp, and I was always slightly annoyed that such a big deal is made over something we can barely see.

In winter, rain collects in shallow depressions of hardpan earth so dense that water can't drain. Every freshwater pool is as unique as a fingerprint, with hundreds of species adapting to precise conditions of its specific environment. An integral element in California's freshwater system, each vernal habitat is like an island, a microcosm that represents our wider ecosystem of interdependence, including specialists and generalists, each with a talent or trick for survival.

Fleeting vernal habitats are easiest to see in the spring. Flowering plants proliferate in concentric rings in a seemingly impossible array of color and pattern. When John Muir walked this valley, he wrote, "Sauntering in any direction my feet would brush about a hundred flowers with every step, as if I were wading in liquid gold."

With summer, the bounty fades in the desiccating heat, and in fall the magic is covered with a sheltering layer of dying matter.

Winter hides what I think are the most fascinating secrets of vernal pools. The land looks barren but softly golden as we drive by. Stop, and an undulating countryside transforms into a surreal fantasy-scape.

At the top of a gentle hill at Rancho Seco Recreation Area, hundreds of fuzzy mounds descend in all directions. These are mima mounds, covered with dead grasses highlighted white in the late afternoon sun. Each casts a shadow over slightly greener, flatter areas that should be filled with water reflecting a huge sky. But most pools are dry now. There's been so little rain.

Dry vernal habitats are sanctuaries for survival, concealing astounding diversity, innovation and adaptation. Some bees living near vernal pools live solitary lives. In spring, each female digs natal chambers, like spokes on a wheel, and in each chamber places one egg on one ball of pollen. Next spring, a tiny, furry adult bee emerges and mates just in time to pollinate its particular choice of flower, dig chambers, make pollen balls and start the cycle again.

Deep within the muddy pool, a dormant coyote plant sends up a snorkel to oxygen above, and in spring grows tall, its thistles waiting to do what thistles do in summer.

As for fairy shrimp, their ability to lie dormant for as many as 100 dry years makes them remarkable. All they need is a little rain.

Vernal pools represent a chain of energy, with decomposers, producers and consumers. They are incubators for an interdependent system of life and food, from expended matter and algae, to a pocket gopher and the majestic great egret. One vernal pool can provide a child with a hands-on science lesson that will last a lifetime.

This valley is one of only five extreme Mediterranean climates in the world, with wet winters and long, dry summers. Of those five climates, California boasts the most spectacularly diverse display of vernal habitats, and nobody knows why.

Since John Muir explored our state, 90 percent of vernal pools have disappeared. What's to be done with the last 10 percent?

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