Jelly Mine

The aptly named Jelly Mine is a freshwater filter-feeder with a hidden danger. Their bodies consist of 40% sodium, encased within an oily liquid a thin jelly-like bubble. They're harmless themselves, but highly explosive when trodden on by animals/people or crushed by vehicles. In this scenario, their jelly exterior breaks exposing generous quantities of sodium to water. This metal reacts violently with water, and the explosive consequences are likened to that of a miniature landmine.