The central octagonal tower, with its vast internal open space and its pinnacles and lantern above, forms the most distinctive and celebrated feature of the cathedral. However, what Pevsner describes as Ely's 'greatest individual achievement of architectural genius' came about through a disaster at the centre of the cathedral. On the night of 12–13 February 1322, possibly as a result of digging foundations for the Lady Chapel, the Norman central crossing tower collapsed. Work on the Lady Chapel was suspended as attention transferred to dealing with this disaster. Instead of being replaced by a new tower on the same ground plan, the crossing was enlarged to an octagon, removing all four of the original tower piers and absorbing the adjoining bays of the nave, chancel and transepts to define an open area far larger than the square base of the original tower. The construction of this unique and distinctive feature was overseen by Alan of Walsingham. The extent of his influence on the design continues to be a matter of debate, as are the reasons such a radical step was taken. Mistrust of the soft ground under the failed tower piers may have been a major factor in moving all the weight of the new tower further out.