If you're lucky. Generally, in England and Wales, if a find is classed as "treasure", it will end up in a museum, though the finder might get a cut of the proceeds. If it's not treasure - and that's decided, bizarrely, by a coroner at an inquest - then the normal rules apply, and if you own the piano you will own what's inside. On 16 March an inquest is re-opening to decide whether gold coins found in a piano in Shropshire in 2016 count as treasure.
Does the attack when a man drove a bus into worshippers at Finsbury Park mosque count as terrorism?