. Recently, some models are turbo-charged, resulting in dramatic increase of engine horsepower. Snowmobiles are capable of moving across steep hillsides without sliding down-slope if the rider transfers their weight towards the uphill side, a process called side-hilling. Mountain sleds permit access in remote areas with deep snow, which was nearly impossible a few decades ago. This is mainly due to alterations, enhancements, and additions of original trail model designs such as weight, weight distribution, track length, paddle depth, and power. Technology and design advances in mountain snowmobiles have improved since 2003 with Ski-Doo's introduction of the "REV" framework platform. Most two-stroke mountain snowmobiles have a top engine size of 800 cc, producing around , although some 1,000 cc factory machines have been produced. These may not be as popular as many 800 cc models outperform them because of weight and an increase of unneeded power. Cornices and other kinds of jumps are sought after for aerial maneuvers. Riders often search for non-tracked, virgin terrain and are known to "trailblaze" or "boondock" deep into remote territory where there is absolutely no visible path to follow. However, this type of trailblazing is dangerous as contact with buried rocks, logs, and frozen ground can cause extensive damage and injuries. Riders look for large open fields of fresh snow where they can carve. Some riders use extensively modified snowmobiles, customized with aftermarket accessories like handle-bar risers, handguards, custom/lightweight hoods, windshields, and seats, running board supports, studs, and numerous other modifications that increase power and maneuverability. Many of these customizations can now be purchased straight off the showroom floor on stock models. Trail snowmobiles improved in the past 15 years as well (many of them borrowed from endeavors to produce winning mountain sleds). Heavy "muscle sleds" can produce speeds in excess of due to powerful engines (up to 1,200 cc stock, and custom engines exceeding 1,200 cc), short tracks, and good traction on groomed trails. Sno-cross oriented snowmobiles often have an engine size cap of 440 or 600 cc, but lighter machines with redesigned stances, formats, and weight control have produced extremely fast and quickly accelerating race sleds. Environmental impact The environmental impact of snowmobiles has been the subject of much debate. Governments have been reacting slowly to noise and air pollution, partly because of lobbying from manufacturers and snowmobilers. For instance, in 1999, the Canadian government adopted the Canadian Environmental Protection Act, 1999,