A glacial crevasse. May be at the top where the glacier abuts a wall of solid rock, or may be an ordinary but unusually large free standing crevasse near there, formed as ice on the crevasse's lower side pulls away from slower moving ice above.
Infrequently encountered on hiking trails.
The general rule is if you leave one alone it will return the favor. Bergschrunds are not known to pursue hikers or get into food bags, even carelessly stowed ones.
On the other hand (as a word to the wise), no human ever won a fight with a bergschrund.