ROCHESTER, N.Y. - Two explorers conducting underwater surveys of Lake Ontario have uncovered an aquatic mystery -- a rare 19th-century schooner sitting upright 152 metres under the waves.

Jim Kennard and Dan Scoville located the 15-metre long dagger-board ship unexpectedly this fall using deep scan sonar equipment off the lake's southern shore, west of Rochester.

The ship is the only dagger-board known to have been found in the Great Lakes. Kennard said vessels of this type were used for a short time in the early 1800s. The dagger-board was a wood panel that could be extended through the keel to improve the ship's stability. The dagger-boards could be raised when the schooner entered a shallow harbor, allowing the boat to load and unload cargo in locations that would not otherwise be accessible to larger ships.

The shipwreck was found upright and in remarkable condition considering it had plunged more than 150 metres to its resting place on the bottom, the men said.

The schooner's origin is a mystery so far.

The name of the schooner is unknown and there are no documented accounts of a dagger-board schooner sinking in Lake Ontario.

The explorers suspect the schooner was being converted to a barge or other sailing craft by its owners and perhaps broke free from its moorings in the ice or during a violent storm and was carried far out on the lake before it eventually sank.

The men found it on the very last survey run of the season. A faint image of something protruding from the bottom showed up at the very edge of the display screen, and another run was made to obtain a better image and the position of the object.

The two explorers returned to the site two weeks later and used a remote operated vehicle to explore and photograph the shipwreck.

It appeared from the video survey of the shipwreck that the schooner had been stripped of all useable items such as anchors, iron fittings, cabin with contents, and tiller, Kennard said.

During the past several months, the explorers have been seeking help from Great Lakes maritime historians to learn more about the schooner.

The dagger-board schooner is one of the older ships discovered in Lake Ontario and the Great Lakes.

In May 2008, Kennard and Scoville discovered the British warship HMS Ontario, which was lost in 1780. The Ontario is the oldest shipwreck ever found in the Great Lakes and the only British warship of this period still in existence in the world.

There are estimated to have been over 4,700 shipwrecks in the Great Lakes, including about 550 in Lake Ontario.