There are several major hazards in Yellowstone. One is the animals, one is the hot water in the thermal springs.
There are also hazards due to breaking through the thin crust over the hot internal features. You see those signs everywhere. The National Park Service has the following warning on their website:
Scalding Water Can Ruin Your Trip
Yellowstone's thermal features, rare among the earth's wonders, are extremely fragile. Boardwalks and trails protect you and preserve delicate formations. You must stay on boardwalks and designated trails. Scalding water underlies most of the thin, breakable crust. Pools may be near or above the boiling temperature and can cause severe, possibly even fatal, burns.
Keep your children close to you at all times; make sure they understand the danger.
Pets are prohibited in thermal areas.
Swimming or bathing in thermal pools or streams, where water flows entirely from a thermal spring or pool, is prohibited. Where swimming is allowed, swim at your own risk. Thermal waters may contain organisms know to cause infections and/or amoebic meningitis, which can quickly be fatal. Obtain more information at any ranger station or visitor center.
When visiting the hot springs and geysers at Yellowstone, you should heed all warning signs. Although they are beautiful, the hot springs and geysers are extremely hot, so never try to touch or get in the water! People have died and others have been severely burned. Sometimes certain trails are closed, and it is not a good idea to try to walk in a prohibited area. Some of the geysers are just beneath the ground, so you could fall in. Also, there are chemicals that may enter the hot springs and geysers that can make them dangerous to be around. Do not assume that you will be okay if you step off of the paths created for tourists. The paths are known to be safe, but the other areas are not necessarily safe, so don't take the risk!
Thermal features are very interesting and prevalent in Yellowstone. Many of the geyser basins have boardwalks that keep you out of harm’s way but if you encounter a thermal feature outside the typical locations you may not be restricted from hurting yourself. At first glance it may look like a hot tub that may be great for a dip.
Making that mistake would be very costly. Hot springs that one can swim in do exist in Yellowstone but if it is not well noted that the springs are OK I wouldn’t even consider it. The water in these pools is usually at or very near boiling. Even walking near the edges of thermal pools can be hazardous since many of them overhang the pools and if the edge were to collapse you may be sent into the pool.
Burns inflicted by thermal features can be serious and possibly fatal. Many people and animals have died from them.
Despite warning signs all over the place that the water from geothermal features may be boiling hot, it is amazing to see how many people cannot resist sticking their fingers into it. I stood next to a group when they stopped in front of the sign, read it, and then one of them said: "It's running water, how hot can it be?!?", and put her hand into it. She was lucky that that particular flow was only very warm, but not scalding.
So even though the water does look deceptively fresh and cool, please heed the warnings and remember that they are called "hot" springs for a reason.
I hope that this story will convince you that the information about fragile ground is true. This story, titled YNP Visitor Burned Near Old Faithful, was in our local Cody Enterprise paper, issue October 13, 2004. A 39-year old Yellowstone Park visitor from Georgia was burned Oct. 7 after breaking through the crust of a thermal area near Old Faithful. The article goes on to explain how the man and a friend had been visiting the Firehole Lake Area when they decided to get off the boardwalk. The victim broke through the crust of what looked like solid ground, submerging both of his legs up to the knees in hot water. His friend pulled him out immediately, but the man suffered second degree burns to 25 percent of his body. His friend drove him to Old Faithful Inn, where the man had to be flown by helicopter to a medial center in Idaho Falls. So please obey the rules to stay on trails, and believe it when you read that the ground in the hot springs and geyser areas can be fragile.
Swimming or bathing in thermal pools/streams whose waters originate from a thermal spring/pool is strictly prohibited. Their waters are frequently near or above the boiling point and since the crust around them are thin, could easily break and you could fall in (people have died). Take the following precautions and you should be safe: stay on boardwalks and designated trails, keep your kids close to you at all times and keep pets on leashes.