Hydrothermal activity results when surface water seeps down to meet the heat of the earth's molten rock, in some places this may be only three miles below the surface. In parks where there are thermal pools, especially in Yellowstone National Park:

• Stay on boardwalks and designated trails.
• Watch for frosty and icy trails and boardwalks, especially in the morning.
• Scalding water underlies thin, breakable crusts. This thin crusts around thermals may suddenly break. Scalding hot water inflicts serious—possibly fatal—burns.
• Pools are near or above boiling temperatures.
• Be especially careful with small children, who seem irresistibly attracted to water and mud. Keep children under control.
• No pets are allowed in the thermal areas.
• Swimming or bathing in thermal pools or streams whose waters flow entirely from a thermal spring or pools is also prohibited in national parks.

Each year, visitors off trail in thermal areas have been seriously burned, and people have died in the scalding water.

Thermal features are easily destroyed. Visitors to the national parks, or any thermal area, for that matter, throwing objects into these features have damaged a number of geysers and hot springs. Walking on them, carving or defacing them, or removing souvenir pieces of formations destroys decades or centuries of intricate natural processes. It is illegal to throw objects into features, deface them, or remove natural features from any national park.

The major factors that contribute to the colors in a hot spring or thermal pool are the water's temperature, minerals, algae, selective reflections, absorption of light by colloidal particles and water itself, and combinations of the above. Colors range from orange to blue to black.

• Geysers, Fumaroles and Hot Springs
• Northwest Hot Springs

A special kind of hot spring, a geyser periodically erupts violently as pressure mounts in the large volume of hot water stored deep under the surface. Some geysers erupt every minute, and others are inactive for months or even years.

• The Geyser Observation and Study Association

Another type of hot spring, a mud pot forms as heated water mixes with clay and other undissolved mineral matter. Depending on the minerals present, it can be quite colorful, earning it the nickname of "paint pot."

• Mud Pot—Yellowstone National Park

Water vapor and other gases expelled from holes in the ground create a kind of steam vent or "dry geyser." Often the expulsion is so forceful that the ground trembles, producing a strong roaring sound like thunder.

• Fumarole
• USGS Photo Glossary of Geologic Terms: Fumarole

Thermal waters may contain microscopic organisms (Naegleria fowleria) known to cause infections and/or amoebic meningitis which can be quickly fatal. Swim at your own risk; do not dive or put your head into springs or streams.

• Meningitis
• Meningitis and Other Central Nervous System Infections