I watched a show today where I found out that for a few days each year part of the banks of the Thames River in London is open to the public to look around.  The river was used as the dumping ground for trash for many years and so now there are apparently quite a few historical artifacts – most of them probably small – on the river bed bottom or on the banks.  Unfortunately I was unable to find information on this event (e.g. the date of the event) on the web.  If anyone has a site which describes when this occurs each year, please post it here.