Mount Tambora today has a massive caldera over 4,000 feet (1,200 meters) deep and about 4.5 miles (7.2 km) across. The mountain was once over 14,000 feet (4,200 meters) tall and the top few thousand feet were obliterated in the massive eruption of April 10th, 1815. 
By far the largest volcanic eruption in modern times
Mount Tambora is a massive stratovolcano that forms the entire Sanggar Peninsula on the northern part of Sumbawa Island in Indonesia. The eruption of April 10th, 1815 is the largest known to have occurred anywhere on Earth during the Holocene Period (11,700 years ago to the present).
Volcanic eruptions are measured on the Volcanic Explosivity Index (VEI) in accordance with how much material is ejected. The 1815 eruption of Mount Tambora erupted about 150 to 160 cubic kilometers of material. For perspective, the major eruption of Mount St. Helens in Washington, USA in 1980 expelled one cubic kilometer.
The Tambora eruption rates a 7 on the VEI. Only supervolcanic eruptions rate higher. Level 8 is an eruption in which 1,000 cubic km of material is ejected, and the last time this happened was when the Taupo Supervolcano on the North Island of New Zealand did it 26,500 years ago.
VEI 7 eruptions are rare, although not as rare as the supervolcanic VEI 8 eruptions. During the Holocene Period there have been at least three other VEI 7 eruptions, although Tambora was the most powerful of these.
This article uses some volcanology terms such as stratovolcano, caldera, and more. For definitions and explanations, see Types of Volcanoes and Volcanology Terms.
The large caldera, which is about 4.5 miles (7.2 km) across, is clearly visible in the middle of Sanggar Peninsula, which is actually all part of Mount Tambora, which also extends further beneath the ocean. 
Effects of the 1815 eruption of Mount Tambora
The massive explosion was heard in locations on the Indonesian island of Sumatra well over 1,000 miles (600 km) away. The amount of energy output was approximately equal to the detonation of 800 megatons of TNT, which would rate about 9.15 on the Richter scale.
Pyroclastic flows of hot ash and gases reached the sea on all sides of the peninsula. These flows generally travel at hundreds of miles per hour and destroy everything in their path. Some villages on this part of the island were obliterated. An estimated 10,000 persons died due to these pyroclastic flows.
The eruption column reached the stratosphere at a height of more than 27 miles (43 km). All vegetation on the island was destroyed. Tsunamis struck nearby islands, killing an estimated 4,600 persons.
Ash fell heavily on the island and on other nearby islands for two weeks, and in lesser amounts for much longer after the primary explosion. Smaller explosions continued for another three months. Ensuing famines on Sumbawa Island and other nearby islands resulted in at least 50,000 to 70,000 additional deaths.
The mountain prior to the eruption stood an amazing 14,100 feet (4,300 meters) tall, and today reaches 9,350 feet (2,850 meters), with a caldera over 4,000 feet (1,200 meters) deep and about 4.5 miles (7.2 km) across.
Estimated volcanic ashfall amounts from the 1815 eruption. The red areas show that the outermost region (1 centimeter thickness) reached the major Indonesian islands of Borneo and Sulawesi. 
Volcanic winter and the Year Without a Summer
The amounts of material put into Earth’s atmosphere by the eruption caused the following year to experience the coolest summer other than in 1601 (due to a massive volcanic eruption in South America) since about 1400 AD.
Significant agricultural problems occurred worldwide. In June 1816 there was frost in Connecticut, and snow fell in Albany, New York and in Maine. Most agricultural crops in North America were ruined, and many livestock died as well the following winter, because the cold was worse than usual.
The climate disruption is blamed for a typhus epidemic in Europe and the Middle East that occurred between 1816 and 1819. Indian monsoons didn’t occur how they normally do, and there were failed harvests and massive famine in India and nearby countries. This in turn has been blamed for causing a cholera epidemic.
Famines occurred in Europe, and food prices rose sharply. There was rioting in many European cities.
The true death toll caused by the 1815 eruption of Mount Tambora is likely at least in the hundreds of thousands.
Breathtaking views of Mount Tambora and its caldera:
Could it happen again?
Certainly an eruption as large or larger will happen on Earth again. In fact, many will but they typically happen every few thousand years. It’s unpredictable. One could happen today or tomorrow, or in ten years, or in thousands of years. No one knows for sure.
Mount Tambora itself has had three minor eruptions since the big one, the most recent of which occurred in 1967. The volcano is not likely to have a massive eruption like what happened in 1815 anytime soon, although it is still an active volcano and a potential danger to nearby areas. Because of this, it is closely monitored – although despite monitoring, volcanoes sometimes surprise everyone and erupt without any warning whatsoever.
These are the world's three true supervolcanoes, which have produced eruptions classified as level 8 on the VEI and which are thought capable of doing it again someday. Toba on the island of Sumatra is about 1,500 miles (2,400 km) northwest of Mount Tambora.
The supervolcanoes are an even bigger worry
As severe as the effects of Mount Tambora’s 1815 eruption, supervolcanoes can do much worse. Their eruptions tend to occur with tens or hundreds of thousands of years between eruptions, and as mentioned above there has not been a true VEI 8 supervolcanic eruption since 26,500 years ago.
The only natural disaster that can top the damage to life on Earth caused by a supervolcanic eruption is a massive asteroid or comet impact. And due to the heavy reliance of humans on electricity and technology, our species is also heavily vulnerable in the event of a super massive coronal massive ejection from the sun, which could knock out electricity over a very wide area and unleash chaos.