Right now, a huge object worth a million dollars is somewhere in the ocean – and according to the law of the sea, it could be yours if you can find it.

The bounty in question is a 1,400-tonne ghost ship, called the MV Lyubov Orlova. In February 2013, this 100-metre long derelict ocean liner was accidentally lost en route to the Dominican Republic after a tow line broke. It drifted off into the Atlantic without a crew or a tracking beacon – and disappeared.

It sparked a global hunt that I described for New Scientist magazine, involving coastguards, satellite providers and even a team of Dutch salvage hunters hoping to cash in. The ship is still out there – possibly sunk; possibly still adrift. This week, the British media reported that it is carrying “disease-ridden cannibal rats” and on a collision course with the UK shore. The first part probably isn’t true, but it could still pose a threat to European coastlines or oil rigs, because prevailing currents are carrying it towards Ireland or Scandinavia.

But how could it be possible to lose such a big ship? In an age of global surveillance, why don’t we have the technology to spot it?

Abandoned boats are not as rare as you might expect. After the Japanese tsunami in 2011, a fishing boat turned up off the coast of the western US – the Navy used it as gun target practice, dispatching it to the ocean floor. Sailboats regularly appear floating and empty – one of the most mysterious was the Bel Amica, which was found in 2006 in the Mediterranean with half-eaten meals. And some have even drifted for decades unnoticed – in the early 20th Century, the Baychimo was abandoned in the Arctic then spotted several times over the following 38 years.

The Orlova, though, is particularly big. At 100-metres (328-feet) long, it boasts a restaurant, a gym and could carry 180 people. Until a few years before it disappeared, it was used for polar tourist trips.

Despite this, its bulk is dwarfed by that of the open sea. When the Orlova disappeared, the software that coastguards use to stage search and rescue missions told them that the prevailing ocean currents were taking it to Europe. But as the days and months have ticked by, the potential search area has become too big to send out ships or planes to look for it.

A group of Dutch salvage hunters have tried though – last year they made two trips into the Atlantic with their scuba diving vessel, zipping back and forth over the sea with a helicopter. Maritime law essentially says that if you throw a rope on a derelict vessel, then it’s yours (or at least the owner must pay a substantial release fee). Unluckily for them, they were struck by engine trouble and bad weather at sea, and were forced to give up their hunt.

You might think a camera on one of the many satellites orbiting the Earth would be a better option. But unless you know where to look, the resolution of the cameras over the ocean is too low to see a ship. This is a big problem for coastguards and the navy. From quota-dodging fishing vessels to pirates, governments and coastguards are desperate to develop better ways of keeping track of what happens at sea. It’s still a Wild West out there.

There is another type of satellite technology that could help. One of the most promising options for tracking the ship – employed by the Irish coastguard fearing an incoming vessel on their shores – is to turn to “radar” and “automatic identification system” (AIS) satellites. First, radar satellites comb the sea surface in great swathes, and all the vessels captured show up as blips like on a ship radar screen. Then, you cross-reference this data with AIS satellite maps, which show the positions of all the active ships on the ocean (see them right now, here). This process could, in principle, reveal a single derelict ship. Why? The Orlova’s beacon was switched off; it’d be the only radar blip not broadcasting.

Despite its best efforts, the Irish coastguard has so far failed to find the ship using this method. They did, however, spot an illegal fishing vessel near Scotland – so there was some silver lining. The hope is to use this network, called C-Sigma, for future maritime surveillance.

However, the humbling truth is that there are still vast swathes of our planet’s surface in which it’s surprisingly easy to lose things. Even a ship the size of a large building. If anything, the Orlova’s story serves as a reminder of our own scale in the world.

If you would like to comment on this, or anything else you have seen on Future, head over to our Facebook or Google+ page, or message us on Twitter.