Outcrop coal was already used as fuel during the Bronze Age (3000 - 2000 BC), where it has been detected as forming part of the composition of funeral pyres in Britain. In the Roman Empire, coal was used extensively in many branches of technology, for example for drying grain and iron working. It was readily available until the Middle Ages, when virtually all surface coal in Europe was exhausted. In later times, coal was mined from underground deposits and used extensively as the main energy source, delivering power to whole cities. Coal and coal byproducts were also crucial for material engineering.