Electron solitary holes were observed in a magnetized collisionless plasma. These holes were identified as Bernstein-Green-Kruskal equilibria, thus being purely kinetic phenomena. The electron hole does not damp even though its velocity is close to the electron thermal velocity. Two holes attract each other like particles of negative mass, and coalesce when their relative velocity is small.
Physical Review Letters, 1979, Vol 42, Issue 8, p. 501-504