Ice diving is a type of penetration diving where the dive takes place under ice. Because diving under ice places the diver in an overhead environment typically with only a single entry/exit point, it is considered an advanced type of diving requiring special training (although whether it constitutes technical diving is part of a wider debate within the diving community). Ice diving should not be attempted by anyone not trained (or in training) by a qualified instructor.
This special training includes learning about how ice forms, how to recognize unsafe ice conditions, dive site preparation, equipment requirements, and safety drills. Ice divers are tethered for safety. What this means is that the diver is wearing a special harness under his/her scuba unit. A line is secured to this harness, and the other end of the line is secured to the surface by one of a number of methods.
Ice diving is a team diving activity because the divers line requires a line tender. This person is responsible for playing out and taking in line so that the diver does not get tangled. Communication to the diver, or to the surface, is accomplished by pulling on the line. Each series of tugs means a different thing. There is a diver suited up and ready to enter the water at a moment's notice. This diver is a safety diver, and has his own tender. His purpose is to assist the primary diver in the event of a problem.