Comets aren't known for being gargantuan, but there are clearly exceptions to that rule. Researchers using the Hubble Space Telescope have spotted the largest known comet to date, C/2014 UN271 (Bernardinelli-Bernstein). With a nucleus 80 miles across, it easily overshadows the 60-mile girth of previous record holder C/2002 VQ94 — it's about 50 times bigger than the typical comet.
The comet was first discovered in 2010 by its namesake astronomers Pedro Bernardinelli and Gary Bernstein. However, scientists only recently verified the size by comparing Hubble imagery against a computer model of the coma (the 'atmosphere' of the comet as it releases gas) and data from the Atacama Large Millimeter/submillimeter Array. At roughly 2 billion miles away from Earth, C/2014 UN271 is too far away for Hubble to visualize the nucleus.
And before you ask: no, there's no danger of an Earth-shattering collision. C/2014 UN271 is on a 3-million-year-long elliptical orbit that will take it no closer than 1 billion miles from the Sun, or slightly beyond Saturn's distance, in 2031. It appears to have originated from the Oort Cloud (the still-theoretical nest of comets at least 2,000AU from the Sun) and may travel up to half a light-year away. Its -348F temperature may seem frigid, but it's warm enough to produce a carbon monoxide coma.
The size confirmation isn't just about bragging rights. This finding widens humanity's understanding of comet sizes, and adds to the still-small catalog of very distant comets. It might also provide more evidence of the Oort Cloud's existence and, by extension, help explain the cloud's role in Solar System development.