AVON PARK, Fla. -- The latest scavenger hunt takes you underwater.

Geocaching started as a hobby more than a decade ago on land but it’s slowly becoming popular with scuba enthusiasts around the world looking for new underwater adventures.

“People are adding this on as an extension to their own hobby,” said Jeremy Irish, CEO and co-founder of Geocaching.com based in Seattle.

Interest in geocaching has grown significantly over the years, even reaching space via the International Space Station. Tourism boards and local parks use geocaching as a way to attract visitors. But combining the two hobbies – geocaching and scuba diving – has only recently taken off.

Underwater geocaches can be found in the waters of more than a dozen countries, from Indonesia to South Africa to Spain, but “there’s just not that many of them out there right now,” said Chris Waggoner, a veteran police officer from Gainesville and an avid geocacher. About 100 geocaches around the world today require scuba gear, according to the Geocaching.com database, and just over half of them were listed in the past two years.

Geocaching is a hunt for a hidden “geocache,” basically a container with a logbook. Some caches hold a trinket or treasure. The person who finds the treasure must sign the logbook and return the geocache to its original location. If you take something from the cache, you must leave something of equal or great value. Geocaching coins and so-called “travel bugs” (dog tags) have tracking codes on them, so they can be moved from cache to cache.

Scuba or underwater geocaching follows the same rules as the terrestrial-based game, but includes some challenges. Land-based geocaching uses GPS coordinates. With underwater geocaching, visual clues are added because it’s difficult to give an exact GPS location on the water. The cache must also be submersible and must withstand water pressure and corrosion; logbooks are waterproof.

Geocaching experiences are shared online on sites such as Geocaching.com or OpenCaching.com run by Garmin, which makes GPS devices. Clues and rating systems are added, including the level of difficulty in finding the cache. For example, does a land hunt require biking or mountain climbing; does a water hunt require special equipment such as scuba gear?

One cold Saturday morning, Kenny Jenkins of Fort Myers placed a cache 33 feet underwater in Lake Denton in central Florida. A homemade red-and-white floating tube bearing the words “Official Geocaching Game Piece” marks the spot, attached to a concrete block that sits on the lake bottom. To prove that they’ve found it, geocachers must sign the waterproof logbook at the site, “because otherwise you have people that just drive by and say ‘I found it’ and they never really did,” said Jenkins.

Jenkins, who has been scuba caching for six months, was excited to add an underwater cache since so few are near his home. “Because they’re fairly extreme, they take a lot of work, a lot of effort to get to, and also to place,” he said. “But a lot of people are doing more outdoor activities now and the geocaching and scuba diving for me combines two of my greatest hobbies.”

Some water caches can be reached by using a canoe or kayak because they are placed just under the water surface or near shore. Others are accessible by snorkel or an easy dive. The more intense searches, though, require scuba gear. One cache, called “The Boss,” can be found near a dive wreck in Cape Town, South Africa. The “Underwater Bus” cache is in Fantasy Lake Scuba Park in North Carolina about 28 feet underwater, and the “Diver’s Underwater Shrine” is submerged 24 feet near the shoreline of Petoskey, Mich. Directions say if you catch the sun just right, you might see the white marble crucifix resting underwater in Little Traverse Bay.

Geocaching is not regulated beyond what is approved by the community or volunteer reviewers on geocaching sites. Guidelines require permission before placing a cache on private property or on protected federal lands, and caches cannot be buried. There’s no age requirement, so families can go hunting together.

There is no record of accidents or deaths associated with underwater geocaching. But there have been deaths among land-based geocachers from heart attacks and falls.

Underwater geocaching is one part of a big hobby, and geocaching overall is growing. Geocaching.com started in 2000 with 75 caches worldwide and now lists over 1.6 million caches.

“I always attributed it to the fact that people generally enjoy exploring and discovering new places,” Irish said. “You get to be a kid again and go treasure hunting.”