Very narrow regions in the interplanetary medium, with abrupt magnetic field strength decreased to nearly zero, have been observed for a long time (Burlaga and Ness, 1968; Burlaga, 1968), and have been termed magnetic holes by Turner et al. (1977). Two major classes of these magnetic holes have been distinguished: "D-sheets" associated with field rotations (Burlaga and Ness, 1968) and "linear" magnetic holes (Turner et al., 1977; Fitzenreiter and Burlaga, 1978). D-sheets are of particular interest since they might be the interplanetary signatures of reconnection events. Linear magnetic holes, on the other hand, are believed to be pressure-balanced structures. However, to date, no sufficiently high time resolution plasma data are available to study these structures. Magnetic holes near Earth, occurring a rate of about 1.5/day, move past a spacecraft in the time range of 2 to 130 seconds, the median time being 50 seconds. These phenomena would easily be observed by Triana/Plasma-Mag.