Pet black holes
I recall a short story in which the first human explorers on Mars discover an alien structure, in which there is a table holding a strange object. One of the explorers presses the wrong button and the field holding the tiny black hole in place shuts off, allowing it to fall, right through the Martian crust and to it's core and most of the way back out, gathering matter as it went. It oscillated back and forth like this, gradually absorbing more and more of Mars' mass until the planet started to collapse, until the 4th planet was replaced by an orbiting miniscule black hole.
Oops.