Seattle’s Groundspeak was one of the early pioneers in geocaching, a high-tech, location-based treasure hunt of sorts where people seek out hidden containers filled with goodies using GPS devices or smartphones. When Jeremy Irish, Elias Alvord and Bryan Roth started the site Geocaching.com in 2000, there were just 75 geocaches listed. On Saturday, the company surpassed 1.5 million active geocache listings, some of which are especially difficult to find.

According to the company, geocaches have been placed on the International Space Station and at the bottom of the ocean floor.

“The International Space Station Geocache is literally ‘out of this world.’ It orbits the earth at 17000 MPH at an altitude of 250 miles,” according to a description on the site.

UPDATE: Groundspeak is one of those hidden (pun intended) success stories in the Seattle area. A true bootstrapped company, it now employs 60 people and is planning its fourth office space move tomorrow. Amazingly, it made its first revenue — in order to support moving the servers to a hosted environment– by selling 144 donated geocaching T-shirts. The business is still owned by the three founders.

Earlier this month, Groundspeak welcomed hundreds of people from around the world to Seattle’s Fremont neighborhood for the Geocaching block party. You can get a sense of the activities here:

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