They are booms grown so large that they cannot gently deflate and must burst, never to return.
Bitcoin has many of the symptoms of a speculative mania, led by the sheer excitement it inspires in its believers. But it's hard to say what the digital asset's value should be. Like gold, value is in the eye of the beholder. It has no intrinsic value, and while the same is true of banknotes, it has no government standing behind it.
Plenty of people are grappling with the same issue, and the value of bitcoin might best be derived from its absence. To see how this works, look at the odd relationship between gold and Treasury bonds, in this chart from Gavekal Research Ltd. Generally, Treasuries beat gold when people aren't too worried about inflation, while gold wins when there are inflationary concerns. Except at present, both are falling: