However, when total yield of individual test devices is considered, there have been changes of considerable magnitude in the program since 1961. The changes upward have been dictated by need, and were made as experience was gained. The slide shows in a general way the step-up in maximum yields of underground test devices by fiscal years. > ss we” we moved upward in 1964 to Bilby with a yield of about 235 kilotons, then on to Halfbeak in 1966 with about 350kilotons, to Greeley in 1967 with about 870 kilotons, and to Boxcar in 1968 with about 1300 kilotons. The yield figures must be approximations where any appreciable fusion contribution is concerned, since post-shot radiochemistry analysis to determine thermonuclear yield is very difficult. It has been the Commission's policy to step up yields on a careful and prudent basis so ground motion effects, in particular, can be extrapolated on a reasonable basis to future events, rather than take a chance of some unexpected and possibly damaging effect coming along with any radical escalation of yield. DOE ARCHIVES Obviously, as yields have increased, depths required for containment have increased also. The first underground tests designed for containment were in mined horizontal shafts. A little later, some so-called safety ex- periments were conducted in drilled vertical shafts not designed for containment, but to increase earth capture of radionuclides. back to 1957 and 1958. weeueneeeetiaien This history goes ie