An acquaintance of mine (Bob Lutz of GM/Chrysler fame) landed his L-39 Albatros gear-up several years ago. It was a particularly expensive gear-up because the L-39 has somewhere on its underside an ice-detection mechanism that uses strontium 90 as an active element. The strontium was spread for 1,500 feet down the runway, which required the involvement of a major hazmat team from the EPA.