While a certain number of whales have been spotted in unexpected places lately, it's fairly surprising that a humpback whale showed up in Boston Harbor this week.

The juvenile whale was swimming near Castle Island (a former fort and now the location of an urban park) in the harbor when a whale watch captain spotted it. Captain Adam Murphy, that person, was piloting a commuter boat for Boston Harbor Cruises from Boston to Hingham and heard about the whale from another boat. The whale was passing the Hyatt Regency hotel, in the proximity of Logan International Airport.

Murphy said he identified the whale as a humpback by its dorsal fin and a "short, bushy blow" of air, according to an article in the Boston Globe.

A spokesman for the New England Aquarium, Tony LaCasse, said that this is the third time since 2013 that a juvenile humpback was sighted swimming in the harbor, as the article confirmed.

Before that, LaCasse said that in his 12 years at the Aquarium, one other young whale ventured that near to Boston. In particular, one famously spent three days swimming between Seaport District and Hull Bay, back and forth. This is the length of the harbor.

Considering that Boston is experiencing winter and many whales have already headed farther south, seeing the humpback this week was "somewhat" unusual, said LaCasse in the article, but Aquarium officials were happy to note that because boating season is over, being near shore is relatively safe for the cetacean.

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-Follow Catherine on Twitter @TreesWhales