"a bursting inward, a sudden collapse," 1829, modeled on explosion, with assimilated form of in- "into, in, on, upon" (from PIE root *en "in").
And to show how entire the neglect and confusion have been, they speak in the same breath of all these explosions, and of the explosion of a mixture of hydrogen and oxygen, the result of which, instead of being a gas or an enlargement of bulk, a positive quantity, is a negative one. It is a vacuum, in a popular sense, because the produce is water. The result is an implosion (to coin a word), not an explosion .... ["Gas-light," "Westminster Review," October 1829]
In early use often in reference to effect of deep sea pressures, or in phonetics. Figurative sense is by 1960.