A valuable hoard of American gold coins has been unearthed in an east London garden — one of Britain's most curious treasure finds.

Buried hoards are discovered every so often, but their Anglo-Saxon, Viking or Roman owners were themselves interred long ago. Whoever hid the 80 coins from the 19th and early 20th centuries may be alive. Why they chose the garden of a residential block in Hackney is a mystery.

Archaeologists more used to deciphering which Roman emperor is depicted on a coin have been taken aback by the find — gold $20 “Double Eagle” pieces dating from 1854 to 1913 and minted mostly in San Francisco and Philadelphia. Estimates put the value at hundreds of thousands of pounds. The coins, so large that each one weighs 33 grams, go on show at the Museum of London tomorrow.

They were uncovered by two residents who decided to do gardening with a couple of friends. A spade hit something hard. Expecting to remove a brick or a rock, they found themselves staring at glistening gold. One finder, interested in archaeology, alerted the Museum of London, which contacted the Portable Antiquities Scheme at the British Museum.

Scheme head Dr Roger Blandtold the Standard: “There is a huge mystery about who might have buried the coins. It's wonderful to speculate. Who buries so many gold coins?”

Today Inner North London coroner Dr Andrew Scott Reid, announcing the find, said the original owner had until next spring to come forward. The finders are remaining anonymous and the find's location is not being released to discourage false claims. An ill-gotten gain has to be possible and police records are being checked.

If the coins are declared Treasure, they will become Crown property and will be valued. Hackney Museum wants to acquire them and the money paid would be split between the land owner and the finders.