(AP) -- When the fall term begins at Pennsylvania's Nazareth Area High School on Monday, one teacher will be conspicuous by his absence.
Matthew Greene was a popular math teacher at the school 65 miles north of Philadelphia when he vanished while on a backpacking trip in California last month.
Police say the 39-year-old Greene was in California to hike and climb the Eastern Sierra, a region that features extremely steep and rugged terrain. The last time anybody heard from him was July 16.
Searches of the vast wilderness area have turned up no trace of the former Peace Corps volunteer.
His disappearance has baffled friends and family who call Greene a highly experienced, cautious outdoorsman.
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