A series of caches based on the letters of the Greek Alphabet. There are 24 letters in the Greek alphabet. They've been around since around 850BC. The Greek alphabet is itself derived from the Phoenician alphabet. The Greek alphabet has itself been the basis of other alphabets including the Latin alphabet.
Modern Greek letters today, are used for symbols in mathematics and science. amongst other things.
"Pi" is the 16th letter of the Greek alphabet.
Pi in its lower case form Π represents the osmotic pressure in chemistry and economic profit in microeconomics, (amongst other things).
The cache is a small lock and lock box placed at ground level.