Ola Oskarsson from the marine survey company Marin Matteknik said the HMS E18 was found last week near the island of Hiiumaa, some 90 miles west of Tallinn, after a 10-year search.
"Apart from the damage on port side and tinier damages elsewhere, the wreck is in surprisingly good shape," Oskarsson said.
The submarine was among British vessels sent to the Baltic Sea during WWI on the order of Winston Churchill — who at the time headed the British Navy — to support Russia in its attempts to stop German shipments of iron ore from Sweden.
The vessel left the Estonian port of Reval, later known as Tallinn, in May 1916 for a routine patrol but never returned. The entire crew, including three Russians serving in a liaison capacity, perished.
Oskarsson said the damage appeared to have been caused by a mine, and that the vessel was probably sailing on the surface at the time of the explosion as the submarine's hatch was found open.
The wreck was found northwest of Hiiumaa in an area known to have been mined by Germany during WWI.
Oskarsson said that although the wreck lies inside Estonia's economic zone, it is in international waters.
"Legally, this wreck belongs to the owner, so only Britain can claim ownership," he said, adding that Britain had been informed the wreck had been found.