The most common type of marker for utilities is that of a paint or chalk mark on the ground. However, there is a growing trend towards the placement of permanent markers around buried utilities, particularly in critical facilities. While tracer wires and buried magnets have been used for many years, more sophisticated permanent marking technology has become available in recent years, specifically, passive electromagnetic ball and radio-frequency ID tags (or ball). These markers act as passive antennas, reflecting back the query signal from the locator without need for an internal power source. Some marker balls feature unique frequencies so that only a particular utility would be indicated by signal reception. These are generally in the HF or UHF range. Passive radio-frequency ID tag or balls (RFID) are similar in concept to passive EM balls, but are also capable of transmitting back data embedded in them such as information about the characteristics of the utility, year of construction and exact spatial coordinates.