Although I'm by nature a geek, by profession I'm a Chemgeek.
This cache celebrates Element #80, Mercury, without which this park wouldn't be here. Mercury is Element #80 in the periodic table. Its chemical symbol is Hg, which comes from the Latin, hydragyrum, meaning liquid silver. Interestingly, the element Mercury was named after the planet Mercury which is named after the god Mercury. Mercury is the only metal that is liquid at room temperature, and is extremely dense. One interesting chemical property is that it will dissolve gold metal and form an amalgam, essentially an alloy of gold and mercury. This property was particularly useful during the California Gold Rush. Finely crushed gold-bearing rock was mixed with mercury to dissolve the gold. The remaining crushed rock was removed, and the amalgam heated to evaporate the mercury leaving purified gold. The problem was where to get the large amounts of mercury required. Fortunately, the hills south of San Jose were long known to contain deposits of cinnabar, the red compound mercury sulfide (HgS). In 1845, predating the discovery of gold at Sutter's Mill by 3 years, the New Almaden Quicksilver mine was opened. By the time it closed, over 76 million pounds of mercury had been extracted from the mine (heating the cinnabar in a process called "roasting" causes the sulfur in the ore to combine with oxygen, leaving mercury metal). Unfortunately, mining and processing mercury is a messy process, and mercury-containing runoff contaminates the streams originating or running through Almaden Quicksilver Park, and passing down to the Guadalupe River to the Bay. (A corresponding contamination occurs in the Gold Country where much of the Almaden mercury was ultimately shipped for use in gold extraction.)
The cache is a large cylindrical jar in a section of this park with few caches. It was about a 20-30 minute hike from the Mockingbird trailhead, so kids should be able to make it with minimal whining (Parents may whine towards the end.) There's a great view by the cache. No poison oak at the cache site, but there is some nearby. FTF Prize is a $10 Starbucks card!