Enterprising internet users in China fear the tools they use to tunnel through the country’s “Great Firewall” may soon disappear, as Beijing tightens its grip on the web.

Tens of millions of people are estimated to use Virtual Private Networks (VPNs) to bypass Chinese internet restrictions — getting access to blocked websites such as Facebook and Twitter.

Beijing has for years turned a blind eye to these holes in its Great Firewall, but recent events suggest the virtual tunnels may soon be bricked up.