FFS, of course it's a pool. built in 1895 in the basement of a mansion. so it looks a little weird by today's standards. the lights on the bottom were novel at the time, as was the fact that it was indoors and heated. the railings on the sides aren't original, added because that's the path you walk on as you tour the house.
it's just an old pool, don't try to make it unnecessarily nefarious.