Several weeks ago, I read an article in The Economist about the Elfstedentocht, a 120 mile long Dutch ice-skating race. The race is held only in years when the canal ice is strong enough to safely hold the 15,000 or so participants -- 300 serious competitors and about 15,000 amateurs. It was last held in 1997.
(The Economist article is here, but I do not subscribe to the on-line Economist, and have used up my three freebies. If you have not, you should be able to read the article.)
An in-depth description of the race can also be found in this conference paper from the Project Management Institution. It's free and quite fascinating!
According to Wikipedia, Elfstedentocht,
There are often points along the route where the ice is too thin to allow mass skating; they are called "kluning points" (from West Frisian klúnje meaning to run on skates over a carpet), and the skaters walk on their skates to the next stretch of good ice. In 1997 ice-transplantation was re-introduced to strengthen weak places in the ice, for instance under bridges. (emphasis added)
What is ice-transplantation, and what are its limits?