How would you cover a hole in the cement basement floor where the washing machine and laundry tub drain into? I assume that there is a pipe that carries the water to an underground sewer pipe. The problem is, you can see the water sitting in the hole and it make the entire basement stink! We would need access to it in case of a clog but it needs to be sealed so the smell cannot leak out.
Any suggestions??

Try some expanding foam, here its called great stuff, get the oil based version of it, it's a red container with yellow top.
Squirt it around the pipe and let it foam up and dry,

Can u supply a pic? Hole in the floor? No hub from a pipe or anything? Sewer gases are very unheathy not to mention combustable, that gas make people very sick you need to address this hole in the floor and have your drains set properly. Did you ask this question in the plumbers forum? Speedball is the plumber you want to listen to, try there and send him a pic