Starting on April 5, 1815 a series of massive eruptions began. The mountain blew its top off on April 10. All vegetation on several nearby islands died. All in all, about 92,000 people died as a direct consequence of this disaster, the largest volcanic eruption in recorded history. About that many more died worldwide as a result of starvation and disease.
See also Krakatoa for an account of another even more famous explosion of a volcano in Indonesia. Krakatoa exploded in 1883.
The eruption sent so much volcanic ash into the atmosphere that weather patterns around the world were altered, causing the following year to be nicknamed the "Year Without A Summer".
Before the explosion, Mount Tambora was about 4000m high; it is now only 2851m high.
Nearly in a right line to the eastward of Java lies the Island of
Sumbáwa, in which stands the volcano of Tomboro, the most violent
in its eruptions of any in the world. One of the most remarkable
occurred in the year 1815, beginning on the 5th of April and
continuing till the middle of July. Its effects were felt over an
immense tract of country, embracing the Molucca Islands, Java, and
portions of Celebes, Sumatra, and Borneo. The concussions produced
by its explosions were sensible at a distance of a thousand miles
all round; and their sound is said to have been heard even at so
great a distance as seventeen hundred miles. In Java the day was
darkened by clouds of ashes, thrown from the mountain to that great
distance (three hundred miles), and the houses, streets, and
fields, were covered to the depth of several inches with the ashes
that fell from the air. So great was the quantity of ashes ejected,
that the roofs of houses forty miles distant from the volcano were
broken in by their weight. The effects of the eruption extended
even to the western coasts of Sumatra, where masses of pumice were
seen floating on the surface of the sea, several feet in thickness
and many miles in extent.
From the crater itself there were seen to ascend three fiery
columns, which, after soaring to a great height, appeared to unite
in a confused manner at their tops. Ere long, the whole of the side
of the mountain next the village of Sang'ir seemed like one vast
body of liquid fire. The glare was terrific, until towards evening,
when it became partly obscured by the vast quantities of dust,
ashes, stones, and cinders thrown up from the crater. Between nine
and ten o'clock at night the ashes and stones began to fall upon
the village of Sang'ir, and all round the neighbourhood of the
mountain. Then arose a dreadful whirlwind, which blew down nearly
every house in the village, tossing the roofs and lighter parts
high into the air. In the neighbouring sea-port the effects were
even more violent, the largest trees having been torn up by the
roots and whirled aloft. Before such a furious tempest no living
thing could stand. Men, horses, and cattle were whirled into the
air like so much chaff, and then dashed violently down on the
ground. The sea rose nearly twelve feet above the highest tide-
mark, sweeping away houses, trees, everything within its reach.
This whirlwind lasted about an hour, and then commenced the awful
internal thunderings of the mountain. These continued with scarcely
any intermission until the 11th of July, when they became more
moderate, the intervals between them gradually increasing till the
15th of July, when they ceased. Almost all the villages for a long
distance round the mountain were destroyed; and it is computed that
nearly thirteen thousand persons perished. By far the greatest part
of this destruction was wrought by the violence of the whirlwind
which accompanied the eruption.Island of Sumbáwa -- Volcano of Tomboro -- Terrific eruption