This is what is alleged:
The News of the World hacked into the phone of Milly Dowler after she was kidnapped and deleted messages that gave her family false hope. The phones of the parents of the 10-year-old Soham girls were hacked after their disappearance. Party leaders and senior political figures from Gordon Brown to Tessa Jowell to John Prescott had their phones hacked. The Royal Family had their phones hacked. Colin Stagg, wrongly accused of the murder of Rachel Nickell, had his phone hacked. Jacqui Hames, former Met police officer and Crimewatch presenter, had her phone hacked. Thousands of others - some famous, many not - all had their phones hacked into by private investigators working for newspapers.
We know that the News of the World, the biggest selling Sunday newspaper in the UK, and its parent company News International, categorically denied any wrongdoing beyond the behaviour of one rogue reporter. News International has said it conducted three in-house inquiries, one of them by an external law firm Burton Copeland, all of which found no evidence of further phone hacking at the paper. This has now been shown to be untrue.
We know that the Metropolitan police have had Glenn Mulcaire's files, in which we learn that there was evidence of Milly Dowler's hacking, since 2006. They have also had access to the files of Steve Whittamore and Jonathan Rees, two more detectives known to have gathered personal details illegally.
We know that the press' own independent regulator, the Press Complaints Commission, carried out two investigations in which it said there was not evidence to suggest phone hacking went beyond one reporter and criticised the Guardian's own reports for being over dramatic.
This is what we don't know:
We don't know who from the News of the World gave detailed instructions to Glenn Mulcaire to hack in Milly Dowler's phone. We don't know who at the paper was responsible for commissioning Mulcaire - and others - to hack into the phones of the the families of murder victims, police officers, and senior political figures. We don't know who were the so-called 'Princes of Darkness' at the paper.
We don't know how far phone hacking went beyond the News of the World. Evidence from the Information Commissioner's inquiry - What Price Privacy? - found that illegal methods of intrusion went beyond News International. Who else was involved in illegal methods of information gathering?
We don't know whether it was limited to hacking phones. A recent Panorama investigation found evidence of email hacking. We have since learnt about bank accounts being hacked. Without knowing what methods were used how will it be possible to prevent them happening in the future?
This is why we may never know:
Right now it looks as though there is lots of activity around phone hacking. There are people like Jude Law and Brian Paddick suing the News of the World - the civil cases. The police are conducting an inquiry, Operation Weeting, that may lead to criminal prosecutions. There are Select Committee inquiries in the Commons.
But there is a good chance all of these could be virtually closed down by November, without disclosure. News International is successfully settling the civil cases and, as part of those settlements, preventing the files associated with them being made public. The police inquiry - much more thorough than any previous - will focus on the individuals being prosecuted. If, as appears likely, these people plead guilty then the court will proceed directly to sentencing and the files associated with the cases will not be made public. The Select Committees have highly constrained remits and limited powers to call for witnesses or papers.
In other words, without a public inquiry we literally may never know. Never know the contents of most of the thousands of Mulcaire phone hacking documents seized by police. Never know the responsibility of senior figures at News International, in the Metropolitan Police, in the phone companies.
This is why we need a public inquiry. Without one we won't know how far this went. We won't know whether it is still happening. We won't know what to do to prevent it happening in future.
Martin Moore is director of the Media Standards Trust and is helping to organise a campaign for a public inquiry into phone hacking - Hacked Off - launching on Wednesday 6th July.
Follow Martin Moore on Twitter: www.twitter.com/martinjemoore