But boffins in Japan reckon they could roam the Earth once again after making a massive breakthrough.
The biological activity detected in the mouse cell eggs included a type of structural formation that reportedly precedes cell division.
The project by an global team took cell nuclei from a well-preserved mammoth discovered in 2011 in Siberian permafrost and placed them into several dozen mouse egg cells.More news: 49ers dealing for Beckham 'still possible'
Scientists recovered cell nuclei from the well-reserved mammoth, named Yuka, which was discovered in Siberia's permafrost in August 2010.
New findings indicate that the resurrection of mammoths is not a fantasy, a research team including members from Kindai University is saying, after cell nuclei extracted from the 28,000-year-old remains of a woolly mammoth were discovered to retain some function.
There were also signs of fix to damaged mammoth DNA.
"Yuka's cell nuclei were more damaged than we thought, and it would be hard to resurrect a mammoth as things stand", said team member Kei Miyamoto, a lecturer in developmental biology at Kindai University.More news: Woman dies after rock thrown from overpass crashes through windshield
"We want to move our study forward to the stage of cell division", said Miyamoto.
Researchers described the activity this week in the journal Scientific Reports.
Miyamoto and his colleagues are working with researchers in Russian Federation to bring mammoths back to life using cloning technology called somatic cell nuclear transfer.
Miyamoto admitted "we still have a long way to go" before the Ice Age species can return.More news: New York attorney general is investigating Trump projects: NY Times