Four <a class="mw-redirect" href="https://en.wikipedia.org/wiki/21_July_2005_London_Bombings" id="mwAWU" rel="mw:WikiLink" title="21 July 2005 London Bombings">attempted bomb attacks</a> by Islamist extremists disrupt part of <a href="https://en.wikipedia.org/wiki/London" id="mwAWY" rel="mw:WikiLink" title="London">London</a>'s public transport system.