Scientists have now concluded that about half of that sea-level rise was due to the thawing of a single, enormous ice sheet in East Antarctica that was once considered stable.

"We estimate from our study that the East Antarctic ice sheet contributed about 10 meters [33 feet] of sea-level rise," said Carys Cook, a geochemist at Imperial College London in the U.K. and the first author of a new study that details the finding.

Studying the causes of the ancient sea-level rise could help scientists understand and prepare for sea-level rises caused by current global warming. (Also see "Why Ancient Earth Was So Warm.")

"Our study demonstrates that if carbon dioxide levels and global temperatures continue to rise, the East Antarctic