background bump floor friction hard heavy industry iron machine mill plate sheet skid slide steel technology texture tool work ACV aerodynamic aerodynamics air air-cushion air-cushion-vehicle air-pressure amphibious craft cushion drive engines fans flexible flexible-skirt flight high-air-pressure hovercraft hull hybrid-vessel impeller inflated inflated-skirt low-frictional-force MTV multi-terrain-vehicle multiple-terrain on-board pilot propel propellers rise sightseeing skirt skirted specialised-transport speed stationary surface surfaces thrust tourism tourists transport transporting travel travelling vehicle calcium carbonate coiled coiled shells crawling exoskeleton eye stalks eyes garden snail gastropoda helix aspersa herbivorous land snails locomotion logarithmic spiral mantle mollusk movement mucus muscular foot olfactory organs ommatophores radula retractable retracting ripple rippling shell skin Slime slimy snail spirally tentacles terrestrial snails textured surface unsegmented