"In 1970, when I got here, the average interval was 66 minutes," Mr. Hutchison said. "Today it's 77 minutes.

"In the 1970's, it was more regular. Our forecasts had a margin of plus or minus 5 minutes -- now it is plus or minus 10 minutes."

Although defined by volcanic rock and water heated up to 255 degrees Fahrenheit, geysers are fragile.

In Nevada and New Zealand, geysers that once were considered permanent are now extinct because power plants tapped into underground steam and water systems. Geothermal plants have even reduced geysers in Iceland, home to Geysir, namesake of all geysers.

With about 250 active geysers, and numerous hot springs, bubbling mud pots and fumaroles, or vents, Yellowstone encloses the largest geothermal basin on earth. In 1992, scientists became alarmed after a northern neighbor of the park, the Church Universal and Triumphant, drilled a geothermal well and filled a swimming pool.

The church later capped the well and agreed to give up geothermal water rights in return for 11 acres of national forest land and access to surface water from a Yellowstone hot spring. The deal is contingent on Congressional approval of the "Old Faithful Protection Act."

This bill, which has been passed by the House of Representatives, would ban geothermal well drilling in a buffer zone around the 2.2-million-acre park. The only other wells that tapped into the underground veins of pressurized water were scientific research wells that were later capped.