The (ILDSA) has provided authentication observers for swimmers attempting to cross the approximately 35-kilometre (22 mi) North Channel between Northern Ireland and the Mull of Galloway. According to the ILDSA, this was first accomplished in 1947 by Tom Blower. The first two-way crossing was completed by a six-person relay team on 28 July 2015. The North Channel is part of the Ocean's seven, a set of seven long-distance open-water swims considered the marathon swimming equivalent of the Seven Summits mountaineering challenge.