A method of producing a localised concentration of energy includes: creating a shockwave propagating through a non-gaseous medium so as to be incident upon a boundary between the non-gaseous medium and a gaseous medium formed by at least one hole in a barrier separating the non-gaseous medium from a gaseous medium. This forms a transverse jet on the other side of the hole which is incident upon a target surface comprising a depression which is spaced from the barrier in the gaseous medium. An apparatus for producing a localised concentration of energy is also described.