Flexible understructures for GEM vehicles will provide obstacle-crossing capability with extremely low power. At low power the advantage of peripheral jet machines over plenum chamber machines is not so evident. Tests of an open AIRMAT bottom showed good obstacle- and ditch-crossing characteristics. An inflated fabric skirt improves the performance of a jeep over adverse terrain, while its folding capability reduces interference for normal operation. GEM over-ocean operation can be provided with either a soft absorbent form or an inflated structure that does not absorb impacts until a. predetermined value is exceeded.