The archive provides a simple free text search function.
My first search (“dog”) resulted in an extraordinary list of results from the relatively simple [Collie dog barking, exterior] to [Spain: Small zoo & aviary in recreational area, with lots of noise from monkeys when teased, dog barks occasionally & traffic.] I then travelled around the world by “river” starting at [Atmosphere recorded at River Mersey, with occasional hoot from boat] to [Wuhan: Quayside of Yangtze river, with hooters].
The sounds can be downloaded and are released under a non-commercial use license.
Be warned. You may find yourself travelling in time and space and unable to stop yourself!
Link to the archive: http://bbcsfx.acropolis.org.uk/