WHHHHHAAAAAT? (Part 2)

We're slowly cataloging the many oddities of 83 Durant. Here is today's mystery: the plugged hole in the floor of the upstairs bathroom, as portrayed here:

It's a mystery, wrapped in an enigma, plugged with red plastic. It's about 2 1/2 inches in diameter, and a little more than an inch and a half deep. It's right in the middle of the floor in that area (yes, we have hardwood floor in part of the bathroom, which is also a bit odd in itself). I thought it wasn't a big deal until the other day I got the heel of my shoe caught in it. Anyway, we need to find a way to plug it so it's not quite so hazardous. Therefore, I am instituting the first contest of the 83 Durant Street blog. We are looking for both creative and useful answers to these two questions:

1) What is this hole? Why is (was) it here?
2) How should we fill/plug/otherwise deal with it?

Please use the comments section! We appreciate any and all contributions. Your prize will be immortal glory in the blog-o-sphere, and our eternal gratitude, of course.

Oh, in case this helps: the hole is near the hook-up for the washer/dryer that was (at some point in time) also in this bathroom. We've found both a dryer plug, and the washer water hookups in the wall. We have not, however, found a vent for a dryer. So perhaps that's what's going on here. Another thing: there is no radiator in this room. Every other room has a (steam) radiator, but there isn't one here. Instead, there was an electric heat element under the sink. Unfortunately, that doesn't seem to work, so we're using a portable electric radiator, which warms things up nicely. Anyway, these pieces of evidence may help in figuring out the answer to #1. Or they may not.