Although Mars appears cold and lifeless today, it obviously had a past with active volcanoes. That's because there are many extinct volcanoes on Mars. In fact, the largest volcano in the Solar System is on Mars, its called, Olympus Mons.
When you're looking at all of the surface of Mars, captured from space, there are 4 huge volcanoes that you can see in the Tharsis region of Mars. Olympus Mons is the largest, and the other 3 are Ascraeus Mons, Pavonis Mons, and Arsia Mons. While Olympus Mons towers 24 km above the surrounding plains, and spans 500 km across, these smaller volcanoes are merely 350-450 km across and stand 15 km tall.
These are just the largest, but there are a total of 20 named volcanoes across the surface of Mars.
Like the Moon and Mercury, Mars shows no signs that it ever had active plate tectonics. One of the most obvious features of plate tectonics on Earth is long mountain chains, where plates are rubbing against each other. This doesn't seem to have existed on Mars.
Nobody really knows why the volcanoes are Mars are so big, but it probably has something to do with the lack of plate tectonics. Without the movement of plates, hotspots were able to stay in one place and just grow and grow for millions of years.
It's possible that the volcanoes on Mars still active, but they haven't erupted in the last few million years. By counting impact craters, scientists can find out how old a part of Martian terrain is. Regions around Olympus Mons are as young as 100 million years old. Since that's only 2% the age of Mars,the volcano has been erupting for 98% the history of the planet, so why would it stop now?