Taiwan propylene explosions one week ago
Propene gas is flammable and has a higher density ($1.81 \ \mathrm{kg/m^3}$) than air. It therefore easily forms explosive mixtures with air when it accumulates near ground level or, like in this case, inside an underground channel without much ventilation. While propene itself is color- and odourless, it is described in this article that "eyewitnesses reported a smell of gas (...) coming out of manholes". This smell comes from volatile compounds which are added to the gas in small amounts to make the detection of leaking gas easier. However, by the time that the smell was noticed, enough propene had already accumulated underground to exceed explosion limits. 

Detection of underground gas leaks by odour alone is therefore not a reliable means to prevent possible explosions, namely when the gas is heavier than air. It is also not possible to exactly localize the leak when the gas escapes from multiple manholes which are spread over a large area. Gas sensors in the underground pipeline network could provide earlier warnings about leaks, and gas accumulation could be prevented in this case by shutting down the affected pipeline. However, prevention of leaks in the first place (by regular inspections) should still have higher priority.

The MSDS data sheet for propene states that shutting down gas supply and letting the fire burn out while cooling the surroundings with water is the recommended way of extinguishing burning propene. So using water to cool down the roads with the aim to reduce ground temperature was not completely off, but trying to fight the fire directly with water is unadvisable.