Below its junction with the River Sheaf, the tunnels which carry the combined waters under Sheffield Station and the Castlegate area have been celebrated by urban explorers and include a huge cavern known as the 'Megatron'. However, a significant proportion of the Porter Brook is also heavily culverted as it meets the city. The river in the vicinity of Matilda Street was rerouted into a new straight channel between 1797 and 1808, bypassing several meanders. The channel was still open in 1832, as shown by Taylor's map of Sheffield, but when the first Ordnance Survey map of the area was published in 1853, parts of the Porter Brook had been culverted, to allow housing and industrial buildings to be built. This included a bone mill, and later a timber yard, with further sections having been culverted by 1891 to accommodate industrial expansion.