- Bring a light to see into chain wells and man-holes.
- Prepare for a strong current (2 to 3 knots).
- Entry is from upstream shore marker, follow the rope.
- Lock 21 lies along the south bank and is accessible via the weir.
- Upperwork lies at 40 feet with the lock bottom at 60 feet.
- Visibiliy is 8 to 12 feet (on a good day).
- Divers have placed lines from various points to aid in exploration.
- S.O.S. does not accept any responsibility or liability for these lines or their use while diving.
Lock 21, located off the southwestern end of Macdonnell Island, is accessible by land from the Historic plaque. From Highway 401, follow exits to Ingleside or Long Sault to Highway 2. Enter the Long Sault Parkway (fee per car) to Macdonell Island. The Parkway's facilities are operated by the St. Lawrence Parks Commission. Picnic tables are on location and a snackbar, washroom, nature trail and camping facilities are only 4 km away on Woodlands Island.
Guard Lock 21 is located at the upper entrance to the 11-mile long Cornwall Canal which was built to circumvent the great rapids of the Long Sault. It was constructed in 1885/86 as part of a second enlargement to the canal system and is 270 feet long between the gates, 45 feet wide and has 14 feet clearance over the sill. The Department of Railways and Canals operated the lock until 1936 after which the Department of Transport took over when the area was flooded to serve as headwater for the Hydro dam at Cornwall.