Where is the largest concentration of sunken ships in the world, and what caused these ships to sink?

"...perhaps the greatest concentration of sunken ships in the world-the passage of water off Guadalcanal known as Ironbottom Sound. It was here from August 1942 to February 1943 that some of the greatest naval battles of World War II were fought"

https://www.amazon.com/National-Geographic-Video-Fleet-Guadalcanal/dp/6304475284

CHUUK LAGOON’S
LOST FLEET CHUUK, MICRONESIA

On February 17, 1944, American Task Force 58 engaged in Operation Hailstone, dropping over 500 tons of bombs on the Japanese navy. Today, Chuuk Lagoon (also called Truk Lagoon) holds the wrecks of 60 Japanese ships, the largest concentration of sunken ships in the world. The 433-foot Fujikawa Maru is the most famous, an aircraft carrier that sits upright in 30 to 112 feet of water, a gaping torpedo hole in her side. A combination of warm water, prolific marine life, and lagoon currents has acted as an incubator, transforming the WWII hulks—their guns, trucks, silverware, and sake bottles left undisturbed—into artificial reefs.
Details: Most air connections to Chuuk are via Guam. Stay at the Blue Lagoon Dive Resort. Doubles from $130; 011-691/330-2727, fax 011-691/330-2439, www.bluelagoondiveresort.com. Dive operator: Blue Lagoon Dive Shop. Two-dive boat trip, $95 per person; 011-691/330-2796, fax 011-691/330-4307.
Best times: January to April.

I also find:

"The countless number of shipwrecks in Lake Huron draws thousands of scuba divers yearly, especially to the Alpena, Michigan region, which holds an underwater park preserve".

greatlakescollectibles.com/Our-Great-Lakes/lake-huron no longer exists

Hmm.... Sable Island, Nova Scotia appears to have at least 350 RECORDED wrecks. The island, or rather sandbar, is 44km long, and the wrecks are scattered all around it, but most occured directly on the island.

http://users.accesscomm.ca/shipwreck/index5.htm
http://www.cbc.ca/news/canada/nova-scotia/sable-island-shipwrecks-at-the-graveyard-of-the-atlantic-1.2755063

Chuuk Lagoon's wrecks, which include both ships and aircraft, are scattered over about 800 square miles of sea, reef and islands.

Possibly the former Royal Naval base at Scapa Flow where the Germans scuttled their fleet in 1919 rather than handing it over to the British. It's one those places where some companies even managed to make profit retrieving bit and pieces from the sunken wrecks.

This might be one of those "it depends" type of questions, as it does not specify what size of an area we're talking about. Is 60 ships in a 40-mile wide lagoon more "concentrated" than say, the 130-odd vessels in the mile-long cove of Mallows Bay? Does the type of vessel make any difference? Mallows Bay sounds more concentrated to me, given these statements, but we don't really know how the ships are arranged in Chuuk Lagoon. "Scores of warships" doesn't tell us how many or in what proximity at Bikini Atoll, and "perhaps" in the Ironbottom Sound reference makes that a bit of an uncertainty.

Wanted to add this interesting site on Mallows Bay.
[ dnr.state.md.us/naturalresource/winter2001/ghostship.html no longer exists ]

On the Maryland side of the Potomac River just west of Chesapeake Bay, the largest shipwreck fleet in the Western Hemisphere sits half-sunk and decomposing. In the early 20th century, hundreds of U.S. vessels were sent to Mallows Bay to be destroyed and scrapped – and to this day the remains of dozens can still be seen in the shallow water ... The lack of effective oversight was realized when a Congressional report in October of 1918 revealed only 134 ships had been completed. A year and a half into the program, this was well behind schedule. Over 260 ships were less than half-completed, and hundreds more had not yet been started ... Germany would surrender on November 11th of 1918. At that time, none of the quickly-commissioned EFC vessels had yet crossed the Atlantic. To this point, the program had officially approved funding and paid for 731 wooden steamships. While over 130 ships had been completed, only 98 had actually been delivered. Of those, only 76 had been used to carry cargo as intended. Despite the war being over the shipbuilding continued building. By September of 1919 the builders had delivered 264 steamships to the government. By this time the United States had no use for the ships; they were left to rot while the powers that be determined how to re-purpose them.

http://sometimes-interesting.com/2013/04/18/the-ghost-fleet-of-mallows-bay/