Two plaques commemorating the polar explorer Ernest Shackleton have gone missing from a Plymouth dockside.
The first plaque marked the 100 year anniversary of the Endurance expedition which left from Plymouth in 1914.
The second was for the centenary of the Quest, the ship Shackleton died on that also left from the city in 1921.
The Devon and Cornwall Polar Society (DCPS) said it was "distressed and disappointed" and suspected they had been stolen.
Devon and Cornwall Police have been informed.
Julie Ellis, chair of the society, which arranged for the plaques to be installed, said: "It is very sad that we are commemorating a great explorer and mindless vandals have decided to prize them off the granite.
"We know it happened between 28 September and 27 October, but we are trying to pinpoint when exactly it happened.
"It is hard to know why they would be stolen - if it was for scrap metal, then surely no reputable scrap metal dealer would be interested."
She said she believed tools of some sort would have been needed to remove the plaques and that local CCTV was being examined.
A spokesperson from The Box museum and archive, run by Plymouth City Council, said: "We help facilitate the installation of the city's historic blue plaques and are really proud of Plymouth's Antarctic connections, so were extremely saddened to hear that the Shackleton plaques have gone missing from their plinth.
"We're looking into how we can support the Devon and Cornwall Polar Society with replacing them, but, in the meantime, would urge anyone who has any useful information to come forward."