Cell phones do not work on tunels and other undeground contructions without help because many meters of earth, stone, concrete and steel to block the RF signals. To make cell phones to work underground, you need to build some sort of structure that allows signals to pass through so you can access your wireless data. Typically cell phone operators use something like a cell phone signal repeater or access point to extend the range underground.
A cellular repeater (also known as cell phone signal booster or amplifier) is a type of bi-directional amplifier used to improve cell phone reception and address coverage issues. A majority of deployments occur inside buildings where coverage is weakened by attenuation from building materials. This situation is weakening is very big in undeground tructures, where there is typically lots of ground and concretete.
A typical repeater system consists of a donor antenna that receives and transmits signal from nearby cell towers, coaxial cables, a signal amplifier, and an indoor rebroadcast antenna. According to Wikipedia typically repeaters now provide 20 dB – 50 dB gain some models even much more). Standard GSM channel selective repeaters (operated by telecommunication operators for coverage of large areas and big buildings) have output power around 2 W, high power repeaters more.
A repeater system needs a sufficient isolation between donor and service antenna, otherwise the repeater can enter in loop oscillation mode (stops working and causes interference).
Another antenna type used undeground is called “leaky feeder“, “radiating cable” or “leaky coax”. Leaky-line coax technology allows a single run of coax to function as a long, winding “antenna” that repeats signal coverage.While normal coaxial cables use outer conductor shields to minimize RF leakage, the outer conductor of leaky coax contains openings to let a controlled amount of RF signal leak out into the air. Leaky feeder system is used for underground mobile communication in mining and undeground mass transit railways. Leaky feeders are typically connected to cellular system repeaters or dedicated base stations.
For more details read Leaky Feeder Cable introduction and This is how you make phones work 100 feet underground articles.