Definition: In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Input: Why are the mass transit systems elevated?, Context: Beneath the plain lies the Biscayne Aquifer, a natural underground source of fresh water that extends from southern Palm Beach County to Florida Bay, with its highest point peaking around the cities of Miami Springs and Hialeah. Most of the Miami metropolitan area obtains its drinking water from this aquifer. As a result of the aquifer, it is not possible to dig more than 15 to 20 ft (5 to 6 m) beneath the city without hitting water, which impedes underground construction, though some underground parking garages exist. For this reason, the mass transit systems in and around Miami are elevated or at-grade.[citation needed]
Output:
it is not possible to dig more than 15 to 20 ft (5 to 6 m) beneath the city without hitting water