This was originally a swimming pool for the Johnstone family that was converted into an otter pool. By modern standards it looks pretty ghastly (though Mole Hall was at one time the only zoo in the UK breeding North American River Otters - and very successfully too). But appearances can be deceptive. What is not clear from this photograph is that the square hole in the wall at the back led to a large grassy enclosures beyond to which the otters had access at all times, and there was another "pop" hole in the wall to the left that connected, via a short tunnel, to a second grassy enclosure.
