Hello group I am a first time user, so be kind please. I bought my wife a new vanity for our little bathroom 53" x 98". I took out the old vanity & under it of course there was no tile. The new one is much smaller in length so the no tile will show. Also, over the years tiles have been cracking & becoming loose. So I took up all old tile (leaving the ones under the toilet there until completely ready to dig in). What the tiles were sitting on was a mud (concrete) base. I found a large crack running the entire length of the floor. Around where the crack is the concrete appears to be crumbling too. I have not dug out any of the concrete base to see what the substrate is (afraid). What do think I should do here, can I fill in where the crack is or what, my fear is if I did that it would begin cracking again. Lastly, the tile we bought to replace the other tile is double the thickness. So even if I did fill the crack the new tile will be double the height & may not fit under the toilet. Hints & help, much appreciated.