Crossing a frozen body or water for ice skating or other winter sports or cold weather activities is potentially lethal. Freezing water kills a person in minutes.
Safety measures and rescue
Safety measures include having (preferably experienced) company, getting local advice, telling your plans to somebody, and having ice prods, an ice pike, rope and spare dry clothes in a watertight package. Thickness and quality of ice should be regularly verified. Keep your distance from your fellow travellers, so that if you do hit thin ice not everybody will fall through at the same time. Where the ice is thin, the distance will also lessen the stress on the ice.
If somebody falls through the ice, he should turn back where he came from and break the ice as far as possible (getting up on the ice and then falling through again will take the force out of you). Getting up is difficult as the ice is slippery; use swimming kicks, ice prods and the help of others through a rope, coat, oar or similar (the helpers should not come too close; form a human chain if possible). When up from the water, do not stand up, but roll and crawl until on safe ice. Change clothes or get warm shelter immediately. Unless you have trained for this and know you can handle it, call for help immediately, during the procedure. If you cannot get out of the water, hang on the edge of the ice avoiding motion to reduce heat loss, and call for help.
By ski, kicksled or similar, you will be able to get out on very thin ice. If it breaks, you may be a long distance from ice that holds your weight concentrated in a smaller area.
With snowmobiles, measuring ice thickness often enough is not practical. You have to know beforehand where it is safe to drive. Snowmobiles can sometimes cross open water (crazy people make records driving tens of metres on water), but the vehicle is heavy and can break through ice that could easily carry people, so do not try that. Waterways are often the most practical route, but currents make ice thickness vary in ways hard for a stranger to guess. Do not trust routes of reckless locals, or routes that may not be safe any more. People drown every year because of ice breaking under their snowmobile.
Docks, bridges and other structures.
Inlets and outlets, including those of underwater pipes.
Other currents, including those at sounds, capes and shallow areas, and vertical streams caused by water cooling quicker at an ice-free spot.
The breaking zone by the shore, caused by variations in water level; water levels vary at sea, but also in lakes, especially lakes regulated for hydrodynamic power.
Areas with vegetation (e.g. reed).
Areas affected by shipping.
Areas covered by snow at time of freezing.
In the spring thick ice is also unreliable, as the sun weakens it.
Sometimes the appearance of the ice gives hints about weak areas, e.g. new snow gets dark where cracks allow water through. Much experience is needed, though, to interpret the signs reliably.
Many of the weak points are impossible to deduce from ordinary maps and the looks of the ice. Local people who have to go out on the ice also in less secure circumstances (such as people living on islands, seal hunters and ice fishers) usually know from experience and oral tradition where weak spots are likely to form in a particular region.
Strong ice can have cracks too, e.g. caused by thermal contraction. Suitable winds can enlarge such a crack and even cause the ice outside it to drive away from the shore.