He isn't, exactly. Or at least not in a place where you'd want to
go visit his gravesite. Voltaire (real name: Francois-Marie
Arouet) made a lot of enemies in his life because of his
brilliant satires and treatises, especially those that attacked
the intolerance, corruption, and irrationality of the religious
establishment. When he died in 1778 and could no longer defend
himself, those enemies struck back. He was denied burial in
church ground until the abbey in Champagne relented. In 1791, his
remains were moved to a place of honor within the Pantheon in
Paris.

However, in 1814 a group of right-wing religious extremists
decided that burial was too good for someone who had so hurt
their feelings in his lifetime, so they broke in one night, stole
Voltaire's remains, and dumped them in a garbage heap somewhere.
Their craven act wasn't discovered for more than fifty years, so
while the memorial in the Pantheon remains, his sarcophagus is
but an empty shell.

Before his burial, his heart and brain had been removed. While
his brain disappeared after being auctioned off in the late
1800s, his heart is still in France's possession-last time they
checked at least - in the Bibliotheque Nationale, hopefully under
a decent lock and key.

~source: "Just Curious about History, Jeeves"
by E. Barrett & J. Mingo