What we think is the ground state, the vacuum, may not be the true ground state. The “danger” of living in a false vacuum is that, under the proper circumstances the false vacuum can drop into the true vacuum. If the difference in energy between the false vacuum and true vacuum is large enough, then the surrounding space can likewise be tipped into the lower state. It’s worth mentioning that the idea of a false vacuum is wild speculation and that there is no indication, not even a little, that the vacuum of the universe is a false vacuum and not the true ground state. This tiny difference is known as the “vacuum energy” or “zero point energy”, and it’s responsible for things like the Casimir effect.