During the mining process, water must be emptied. But after the mining operation has been abandoned, groundwater is allowed to seep in, and rainwater collects in the quarry. The depth of a quarry lake is dependent upon rainfall in the region.
Water-filled quarries can be very deep, often 50 ft (15 m) or more, and surprisingly cold, so swimming in quarry lakes is generally not recommended. Unexpectedly cold water can cause a swimmer's muscles to suddenly weaken; it can also cause shock and even hypothermia. Though quarry water is often very clear, submerged quarry stones and abandoned equipment make diving into these quarries extremely dangerous. Several people drown in quarries each year.
Quarry lakes, even lakes within active quarries, can provide important habitat for animals.