Its name says it all. Pianosa, roughly translated, means "flat-o-rama." The island is known as the zattera ("raft") of the archipelago, as its flat top doesn't slope down to the water but drops off to the sea abruptly around its coast. This unyielding flatness means that when you're on Pianosa, you can't even see the water until you're right at the edge of the island. Pianosa has Roman ruins, and interesting birdlife (some introduced when this was a hunting reserve of the Grand Duke of Tuscany).
The biggest presence on Pianosa is its palpable history as a penal colony. Italy has had many prison islands over the years -- all but one, on the Tuscan island of Gorgona, are now defunct -- but for its final decades of existence, Pianosa was Italy's Alcatraz, the place where big-time, high-profile criminal masterminds were sent. During the 1970s -- the so-called anni di piombo, or "leaden years" -- when Italian political dissent turned into terrorism, commandos from violent groups like the Red Brigades did time on Pianosa, which by then had been upgraded from a small-time penal colony to a maximum-security facility. Following the horrific 1992 car-bomb assassinations of Sicilian anti-Mafia magistrates Giovanni Falcone and Paolo Borsellino in Palermo, Pianosa was the penitentiary where convicted mafiosi were incarcerated. Prior to Pianosa's transformation into the supercarcere it became, the island had been a quiet colony of mostly smalltime crooks who lived harmoniously alongside civilian residents and support staff. But during those supercarcere years, helicopters buzzed incessantly overhead, sirens wailed, and a long, terrible wall was erected to more meaningfully separate the prison areas from the other inhabited parts of the island.
From the mid-19th century until 1998, the penal colony kept tourism away and the island unspoiled for 150 years. Since the closure of the prison, Pianosa has been wholly part of the Parco Nazionale dell'Arcipelago Toscano, and visits are restricted to guided tours of no more than 100 persons per day, most of whom visit as a day trip from Elba. There are no overnight accommodations on Pianosa, and only one bar/restaurant (at Cala Giovanna), but it makes for a nice day trip from Elba (you can also get there from the mainland port of Piombino, though all ferries from Piombino to Pianosa stop at Elba along the way).