According to Official Guinness Records,
The deepest half marathon race took place at a depth of 212 m (695 ft) in Bochnia salt mine, Poland on 4 March 2004.
The running track was 2,438 m long which the 11 participants circuited approximately 8.5 times to run a total distance of 21,097 m. Bochnia is Europe's oldest salt mine;operating since 1248.
For a complete list of 2004 records, please visit 2004 Guinness Records in Poland.