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One foggy morning in 1884,

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the British steamer "Rumney" crashed
into the French ship "Frigorifique."

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Seeing their ship filling with water,

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the French crew climbed 
aboard the "Rumney."

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But as they sailed towards 
the nearest port,

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a silent form suddenly emerged
from the fog:

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the abandoned "Frigorifique."

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It was too late to turn,

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and the impact was enough 
to sink the "Rumney."

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As the sailors scrambled 
into the lifeboats,

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the empty "Frigorifique"
sailed back into the fog,

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having seemingly taken its revenge.

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In reality, the French sailors had left
the engines running,

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and the "Frigorifique" sailed in a circle
before striking the "Rumney"

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and finally sinking.

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But its story became one of the many tales
of ghost ships,

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unmanned vessels that apparently
sail themselves.

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And although they've influenced
works like "Dracula"

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and "Pirates of the Caribbean,"

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crewless ships aren't the product
of ghostly spirits,

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just physics at work.

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One of the most famous ghost ships
was the "Mary Celeste"

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found sailing the Atlantic in 1872
with no one aboard,

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water in its hold,

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and lifeboats missing.

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The discovery of its intact cargo
and a captain's log that ended abruptly

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led to wild rumors and speculation.

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But the real culprits 
were two scientific phenomena:

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buoyancy

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and fluid dynamics.

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Here's how buoyancy works.

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An object placed in a liquid displaces
a certain volume of fluid.

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The liquid in turn exerts
an upward buoyant force

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equal to the weight of the fluid
that's been displaced.

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This phenomenon is called
Archimedes's Principle.

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Objects that are less dense than water,

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such as balsa wood,

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icebergs,

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and inflatable rafts

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always float.

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That's because the upward buoyant force

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is always stronger than 
the downward force of gravity.

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But for objects or ships to float when 
they're made of materials, like steel,

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that are denser than water,

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they must displace a volume of water
larger than their weight.

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Normally, the water filling a ship's hull
would increase its weight

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and cause it to sink -

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just what the "Mary Celeste's" crew feared
when they abandoned ship.

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But the sailors didn't account 
for fluid dynamics.

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The water stopped flowing at the point
of equilibrium,

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when it reached 
the same level as the hull.

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As it turned out, the weight of the water
wasn't enough to sink the ship

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and the "Mary Celeste"
was found a few days later

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while the unfortunate crew 
never made it to shore.

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Far stranger is the tale 
of "A. Ernest Mills,"

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a schooner transporting salt,

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whose crew watched it sink to 
the sea floor following a collision.

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Yet four days later,
it was spotted floating on the surface.

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The key to the mystery lay in the ship's
heavy cargo of salt.

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The added weight of the water in the hull
made the vessel sink,

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but as the salt dissolved in the water,

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the weight decreased enough

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that the force of gravity 
became less than the buoyant force

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and the ship floated back to the surface.

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But how do we explain the most
enduring aspect of ghost ship legends:

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multiple sightings of the same ships 
hundreds of miles and several years apart?

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The answer lies in ocean currents,

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which are like invisible rivers flowing
through the ocean.

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Factors, like temperature,

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salinity,

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wind,

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gravity,

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and the Coriolis effect from
the Earth's rotation

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create a complex system of water movement.

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That applies both at the ocean's surface
and deep below.

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Sailors have always known about currents,

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but their patterns weren't well known
until recently.

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In fact, tracking abandoned ships was how
scientists determined the shape

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and speed of the Atlantic Gyre,

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the Gulf Stream,

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and related currents in the first place.

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Beginning in 1883, 
the U.S. Hydrographic Office

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began collecting monthly data
that included navigation hazards,

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like derelict ships, whose locations
were reported by passing vessels.

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So abandoned ships may not be moved
by ghost crews or supernatural curses,

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but they are a real 
and fascinating phenomenon

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born through the ocean

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and kept afloat by powerful, invisible,
scientifically studied forces.