It's been many years since I last took the Underground Tour, but as I remember it, there was a windowless room and some very old furniture. At Mario's, a pizzeria a couple doors north of the tour entrance, you can get all of that for free if you buy a slice of pizza.

It's not very good pizza, though. Is there any excuse for mediocre pizza? All you need is a very hot oven, the patience to stretch or toss the crust thin enough, and decent-quality toppings.

Pizza is bread, and great pizza starts with great crust. When judging pizza, I ask whether it would still be good to eat if the same crust were baked with nothing but olive oil and salt. It's a tough but fair test, one that some pizzas in town can pass and others (Mario's included) cannot.

Mario's oven, a conveyor-belt system, doesn't seem to be hot enough to give the pies a proper char, so the crust is a bit soggy and the cheese insufficiently browned. The toppings are less than imaginative, although if you build your own pie you can choose from a wide variety of cheeses including Gorgonzola and cheddar. Preferably not together.

Mario's is a simple storefront opening onto Pioneer Square with a nice outdoor seating area. (Seagull-infested, but that's not the restaurant's fault.) One glass case displays about 10 pizza pies, and another has Dreyer's ice cream. The help is friendly: There are usually two guys behind the counter, one serving and one cooking, both greeting regulars.

The food is unexceptional, but Mario's dining room is something unusual.

It's a large room, much larger than you'd predict on the way into the restaurant, its walls windowless and blank except for some old photos of Pioneer Square, which makes it feel that much more like the Underground Tour. There's an open storage area in back and low wicker chairs. I kept expecting someone to walk out and say, "Welcome to purgatory. We'll need you to sit here for the next few years until we get everything sorted out upstairs. Have some pizza."

There is a second Mario's location on Third Avenue. I haven't tried it, so this review doesn't apply there. Pioneer Square deserves great pizza to go with its abundance of beer (which isn't available at Mario's, by the way), but that day is not yet at hand. If you find yourself in the area with a craving for pizza, I recommend the ultra thin-crust pies at Post Alley Pizza (1123 Post Alley) instead. If you want to scare your tipsy friends, though, sit them down in Mario's dining room and start talking about 19th century plumbing.

Check Please

Slice, salad and soda combo: The "Mario's" slice features pepperoni, Canadian bacon, sausage, mushroom and olive under an oppressive blanket of mozzarella. The salad, some tasteless greens with black olives, is also topped with shredded mozzarella. Why?

Sandwich and soda combo: I chose the open-face Hot Sub, with pepperoni and Canadian bacon on "tangy Italian sauce." The sauce was undetectable, and with a heavy layer of mozzarella, this is a major grease-trap of a sandwich.

Itemized bill, meal for two

Slice, salad and soda combo $4.89

Sandwich and soda combo $5.69

Tax $0.98

Total $11.56

Matthew Amster-Burton: mamster@mamster.net