Scientists at Toronto's Royal Ontario Museum are trying to get to the bottom of a deadly mystery: what killed thousands of their honeybees?

"Our bees died [and] we're very sad because we're supposed to be a science museum and all scientific but we are a little attached to our bees," said ROM spokesman Julian Kingston.

And the scene is a sad one. A once thriving beehive has turned into a tomb for 20,000 bees.

The bees were part of the biodiversity exhibit at the ROM. They were viewed through a special glass hive and staff say there was no sign that anything was wrong. The bees, they say, were perfectly healthy until late last week.

Then, within two days, they were all dead.

Janine McGowan, ROM's head beekeeper for the past several years, says she's been trying to come up with an answer. 

"It's kind of like playing Sherlock Holmes, in a way, where you look for certain symptoms and you can diagnose different problems." 

McGowan has ruled out colony collapse disorder, a mysterious affliction that's been gouging the honeybee populations around the world. In colony collapse disorder bees leave the hive and never come back. But the bees at the ROM just dropped dead.

McGowan's working theory is the deaths were caused by a lack of procreation.

"I believe that actually what happened is that their numbers were too low to survive the winter and they may have indeed either frozen or starved to death," she said.

Scientists say it happens — even to the best of beekeepers.

But for the beekeepers at the ROM the best they can hope for is a better understanding of the problem.

"The unfortunate death of our hive does become a great teachable moment for us because we can educate people about what happens to beehives — what are some of the perils of bee-keeping," said Kingston.

The ROM promises that by March the hive will be buzzing again.