What is a mudlark? A mudlark is a British term referring to one who made their living by searching for coal, bits of copper, or anything else they could sell to get by. This was a profession long ago.
This sign is from a pub near the Thames River in London. The Thames fluctuates around twenty feet between high and low tide. People go out and search the mud for items. They have to pay attention as they search as the tide comes back in quickly.
Today people who mudlark are hobbyists. They are searching for Saxon items, ancient coins, Roman or Bronze Age artifacts, etc. One may find a blackened roof tile from the Great London Fire of 1666 or a clay pipe. Mudlarks of today are required to get permission and report anything of archeological value. This includes items that may qualify as treasure or human remains. Some prison transport ships to Australia left from London docking along the Thames River and some executions long ago took place near the river.
If you walk along the Thames at low tide, you may see some people busy Mudlarking. You could always join them. You never know what you could find.
Information on getting permission to Mudlark: https://www.pla.co.uk/Environment/Thames-foreshore-permits