Lake Bolsena: a crater lake 151 m deep

Lake Bolsena fills an extinct volcanic caldera in Lazio: 114.5¹ square kilometers of water, 151¹ meters deep at its deepest, with the surface at 305¹ meters above sea level.

The numbers, from two agencies

Lake Bolsena has a surface of 114.5¹ square kilometers, a maximum depth of 151¹ meters and a shoreline of 43 kilometers, on figures from the Lazio regional water plan confirmed by the regional environment agency.

It measures 14 kilometers long and 12 wide, close enough to a circle that the shape reads at a glance on any map.

The elevation of 305¹ meters above sea level appears in only one of the two sources. Treat it as the softer number of the set.

A crater, not a valley

The lake fills the caldera of the Vulsini volcanic complex, and it is the largest lake in the world formed in an extinct volcanic crater, according to the Lazio regional water plan.

That world ranking rests on one institutional source, so it's quoted here with its name attached rather than as settled fact. The same plan ranks Bolsena fifth among Italian lakes by area and first in Lazio.

The volcanic origin also explains the depth. A valley lake fills a trough carved by ice or rivers; this one fills a hole the ground left when the volcano emptied.

Two islands, Bisentina and Martana

Bolsena has two islands, the Bisentina and the Martana.

Boat services to either of them, with their season and their fares, are not in the data verified for this site. Ask at the harbor before counting on a crossing, and don't plan a day around an island visit you haven't confirmed with the operator.

Bolsena against Trasimeno

Trasimeno is larger on the map, at 124³ to 128³ square kilometers, and ranks fourth in Italy, one place above Bolsena.

Depth is where the two part ways. Trasimeno runs 3 to 6 meters deep across its basin, while Bolsena drops to 151¹ meters: the difference between a lake with a few meters of water in it and a flooded volcano.

Trasimeno also has a regional park around it, set up in 1995 over 13,200⁴ hectares. Bolsena's protection status isn't among the verified facts here. See Lake Trasimeno and Italian lakes by size.

What the numbers leave out

Nobody in the sources used here publishes Bolsena's water temperature, bathing quality or boat timetable.

Those are the three things a summer visitor asks first, and they're logged as gaps rather than filled with guesses. Check the bathing notices of the lakeside town halls before you swim, and see Lazio for the rest of the region.

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