How can we access plumbing in floor if we tile the floor?

We have a house that was built in 1900s and a bedroom was made into the bathroom. The plumbing is in the floor. Plywood was screwed around the plumbing as an access door and then carpet was placed over it - of course the carpet got moldy after many, many years. Now we want to place ceramic tile in the bathroom - how can we still get access to the plumbing easily if something goes wrong and there is tile there? I want to think you can do a tile an access panel that can be removable but would it leak if it got wet? Help!