U2 have surprised their fans with a mysterious letter in the mail, which contains certain clues to their next big announcement. Whether or not it's a new album coming remains to be seen, but there are certainly many theories abou the surprise behind the notes. 

U2 perform live at the SSE ArenaU2 perform live at the SSE Arena

The letter in question contains a passage from the foreward of William Blake's collection of poems 'Songs of Innocence and Experience', which became the main inspiration behind U2's last album 'Songs Of Innocence'. However, it is partially blacked out by a silhouette of two figures - a man and a woman - holding hands, with a few words left un-blanked creating the sentence: 'Blackout... It's clear who you are will appear... U2.com'.

Then, at the bottom of the letter is the sentence: 'U2 will announce [blank] on [blank].'

Fans have offered various theories on what the stunt is all about. One suggested on Twitter that it was in celebration of Bono and his wife Alison Hewson's wedding anniversary. '21st August is Bono and Ali 35th wedding anniversary that's what think is in the blackout', they wrote.

Another linked the silhouettes to a photograph that was projected onto a large screen at the end of their recent Joshua Tree Tour dates which, rumour has it, is a picture of one of Bono's sons and one of the Edge's daughters.

It doesn't seem like everybody in the online fan club have got a letter, however; so far it seems only a selection of US fans have had one which could suggest the announcement of another leg of the tour, the current dates of which they aren't due to wrap until October.

More: U2 celebrate the anniversary of 'The Joshua Tree'

U2 return for the second US leg of the tour on September 3rd 2017, kicking off at Detroit's Field with Beck supporting them. On October 3rd 2017, they'll hit Mexico City, and complete the already known tour dates in Brazil on October 25th with support from Noel Gallagher's High Flying Birds.