Bubbles in Financial Markets

Bubbles are created when only buyers exist for a particular share and its price inflates. But sometimes the situation can change in no time when all buyers disappear and only sellers are left in the market, that is when bubble explode and share price crashes. But the problem lies in the fact that every transaction done is legal and every risk taken is calculated. So, there is no way one can anticipate a meltdown. Whether there is a bubble in the making or not. And if there is one, then how can it be deflated without causing panic in the markets.