Crazy valuations for untested companies. Fortunes minted over-night. Promises of new technologies that will transform whole industries. To anyone who remembers the dotcom bubble of 1999 and 2000, that will all have a familiar ring to it – and they should be able to recognise the signs that something similar is happening now.

Some of the new internet businesses are achieving are extraordinary valuations. Uber is now worth more than $60bn. Airbnb is raising a new round of cash that will value is at $30bn. Across the world, there are now dozens of “unicorns”, as start-ups worth more than $1bn are known, and they are collectively worth hundreds of billions.

There is one big difference between today and 2000, however. This time around, the bubble is largely invisible. The huge valuations are being achieved in private placings – and not on the listed exchanges. The trouble is, that doesn’t mean it isn’t dangerous. If the bubble bursts – and let’s face it, most do eventually – then the losses will ripple out through the financial system and end up hurting all of us.

The dotcom bubble of the turn of the century has gone down as one of the epic speculative manias of all time. Young guys in chinos and polo shirts could raise millions, and blow it just as quickly. Boo.com managed to burn its way through $188m in a year trying to build a global fashion store before going bust. Pets.com partied its way through $300m of other people’s money before crashing spectacularly. Investors were hurling money at entrepreneurs who promised they could transform the global economy and make billions. A few did. If you bought shares in Amazon even at the height of the mania, you’d have done well. Most, however, disappeared very quickly.