When staff at Wakehurst Place dredged a pond to encourage more wildlife to the site, they only expected to find leaves and silt.
But instead they discovered wartime relics dropped by soldiers based at the site ahead of the D-Day landings.
The historic finds at the gardens in Ardingly, near Haywards Heath, were discovered when work was carried out on the pond.
They are believed to have been thrown or dropped into the water by British and Canadian soldiers stationed at the 16th Century mansion during the Second World War.
Items included tools, cups, a bullet and vehicle battery.
David Marchant, Wakehurst's logistics manager, said he was enthralled to uncover the treasures.
He said: "I'm quite an amateur historian myself so it was very interesting to be there to see these things come out of the water.
"Wakehurst Place was used as a holding base ahead of the D-Day landings. Military vehicles were hidden here under camouflage nets until they were needed and were transported to the coast.
"I have heard the stories about the soldiers being here during the war and sometimes marrying local girls and staying here.
"They had a big influence on the area."
Mr Marchant has a close affinity with Wakehurst Place - he was born there.
His father was appointed head gardener in 1923 and his mother worked as a housemaid in the mansion and it has been his home ever since.
He organises the machinery and operations behind the scenes at the estate.
He said: "We are hoping for somewhere to put them on display, hopefully here at Wakehurst Place. I don't want them to be hidden away where no one can see them.
"It was fascinating to see what turned up and amazing to think that they have lain undisturbed under the water for all that time.
"Thankfully there were no unexploded bombs."
Wakehurst Place is run by the Royal Botanic Gardens, Kew.
More than 440,000 people a year flock to tour its stately mansion and immaculately manicured gardens.