Some 20 crypto currency service providers are registered with Dutch central bank DNB but it is unclear how many people are actually investing in the volatile currencies, news website Nu.nl said on Thursday.

Registration does not mean a seal of approval, a spokesperson for the bank told Nu.nl. ‘It only means that the providers are not involved in criminal activities, but that is as far as monitoring goes.’

Financial services authority AFM  and investors association VEB both say investing in crypto services, in the Netherlands or abroad, is ‘a hype’ but that they have no idea how many people actually own crypto currencies.

‘Trade volume varies widely between crypto platforms,’ Christiaan Jimmink, marketing manager at Bitcoin Meesters, the largest crypto platform in the Netherlands, told the news platform.

‘We did our own research and we estimate that some 700,000 Dutch people invest in bitcoin. We have around 200,000 Dutch clients. Some spend €10, others hundreds of thousands of euros. Our daily trading volume runs into the tens of millions,’ Jimmink said.

Both the DNB and the VEB regard investment in bitcoin as ‘a dark pool’, or trading systems which are not completely transparent.

‘We are not advising for or against investment but we do have an opinion,’ the DNB spokesman said. ‘A crypto does not represent anything, it’s not a share in anything. It’s not a loan which is returned with interest.’

The AFM said it is keeping an eye on developments, including warning consumers about the risks of investing in crypto because of the wildly varying value of the digital currencies.

Bitcoin services are also controversial because mining bitcoin is extremely energy consuming. Recently former bitcoin advocate Elon Musk said his company Tesla would no longer use bitcoin for transactions because of the resulting high carbon dioxide emissions