A double bottom is a ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

 hull

Hull (watercraft)

A hull is the watertight body of a ship or boat. Above the hull is the superstructure and/or deckhouse, where present. The line where the hull meets the water surface is called the waterline.The structure of the hull varies depending on the vessel type...

 design and construction method where the bottom of the ship has two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull which is somewhat higher in the ship, perhaps a few feet, which forms a redundant barrier to seawater in case the outer hull is damaged and leaks.

The space in between the two bottoms is often used as storage tanks for fuel or ballast water, though fuel storage in the double bottom is not allowed for newbuilt ships since 2007, due to MARPOL 73/78

MARPOL 73/78

Marpol 73/78 is the International Convention for the Prevention of Pollution From Ships, 1973 as modified by the Protocol of 1978....

.

Double bottoms are significantly safer than single bottoms.
In case of grounding or other underwater damage, most of the time the damage is limited to flooding the bottom compartment, and the main occupied areas of the ship remain intact.
For this reason, double bottoms have been required in all passenger ships for decades as part of the Safety Of Life At Sea

International Convention for the Safety of Life at Sea

The International Convention for the Safety of Life at Sea is an international maritime safety treaty. The SOLAS Convention in its successive forms is generally regarded as the most important of all international treaties concerning the safety of merchant ships.- History :The first version of the...

 or SOLAS Convention.

An even more extensive protection is available as a double hull

Double hull

A double hull is a ship hull design and construction method invented by Leonardo da Vinci where the bottom and sides of the ship have two complete layers of watertight hull surface: one outer layer forming the normal hull of the ship, and a second inner hull which is some distance inboard,...

, where the second hull layer extends up the sides of the ship as well as in the bottom.

A double bottom also conveniently forms a stiff and strong girder

Girder

A girder is a support beam used in construction. Girders often have an I-beam cross section for strength, but may also have a box shape, Z shape or other forms. Girder is the term used to denote the main horizontal support of a structure which supports smaller beams...

 or beam

Beam (structure)

A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...

 structure with the two hull plating layers as upper and lower plates for a composite beam.
This greatly strengthens the hull in secondary hull bending and strength, and to some degree in primary hull bending and strength.