German police say shutting down the infamous darknet site took months of cyber investigation

"It gave us all goosebumps" says Sebastian Zwiebel, as he recalls the moment his team shut down Hydra, the world's largest darknet marketplace.

The website was a bastion of cyber-crime, surviving for more than six years selling drugs and illegal goods.

But, after a tip-off, German police seized the site's servers and confiscated €23m (£16.7m) in Bitcoin.

"We've been working on this for months and when it finally happened it felt big - really big," adds Mr Zwiebel.

Police say 17 million customers and more than 19,000 seller accounts were registered on the marketplace, which now carries a police seizure notice.

Hydra specialised in same-day 'dead drop' services, where drug dealers (vendors) hide packages in public places before informing customers of the pick-up location.

Shortly after the German action was announced, the US Treasury issued sanctions against Hydra "in a coordinated international effort to disrupt proliferation of malicious cybercrime services, dangerous drugs, and other illegal offerings available through the Russia-based site."

In the past six months, many high-profile darknet markets have shut down but Hydra was seemingly impervious to police attempts to stop it.

The website launched in 2015 selling drugs, hacked materials, forged documents and illegal digital services such as Bitcoin-mixing - which cyber-criminals use to launder stolen or extorted digital coins.

The site was written in Russian, with sellers located in Russia, Ukraine, Belarus, Kazakhstan and surrounding countries.

Mr Zweibel says the operation to close it down began with a tip-off which pointed to the possibility that the website infrastructure might be hosted in Germany.

"We got some hints through monitoring darknet activity from US officials. So we started in July or August last year to dig deeper and to investigate this field," he says.

It took many months to locate which firm might be hosting Hydra in Germany. Ultimately it was found to be a so-called 'bullet-proof hosting' company.

A bullet-proof hosting company is one that does not audit the websites or content it is hosting, and will happily host criminal websites and avoid police requests for information on customers.

Mr Zweibel says his investigators then took their evidence to a German judge to get permission to approach the server company and issue a takedown notice.

The company was forced to comply otherwise they too could have been arrested.

Visitors to the site are now greeted with a police poster saying "the platform and the criminal content has been seized".