Pea Pod and Boo are correct, the ones you use around the house, from cleaning floors and windows, to using in the bath or shower, are the ones that used to be alive at the bottom of the sea.
While it is true that real sea sponges have been in use since the Roman Empire, synthetic alternatives made primarily from wood pulp became commonplace by the middle of the 20th century when DuPont perfected the process of manufacturing them. Today most of the sponges we use are made from a combination of wood pulp (cellulose), sodium sulphate crystals, hemp fibers and chemical softeners.
Some forest advocates decry the use of wood pulp for producing sponges, claiming that the process encourages logging, but the manufacture of cellulose-based sponges is a pretty clean affair. No harmful byproducts result and there is little waste, as trimmings are ground up and recycled back into the mix.
Another common type of artificial sponge is made of polyurethane foam. These sponges excel at cleaning but are less ideal from an environmental perspective, as the manufacturing process relies on ozone-depleting hydrocarbons (set to be phased out by 2030) to blow the foam into shape. Also, polyurethane can emit formaldehyde and other irritants and can form cancer-causing dioxins when incinerated.