that the majority of the main summits of the massif were first attained. Members of the England-based Alpine Club were instrumental in many of the first ascents, usually accompanied by guides from Chamonix or Courmayeur, such as Michel Croz, Michel Payot and Émile Rey, as well as the Swiss guide, Christian Almer. Across the massif there are now more than two thousand different mountaineering routes to the summits, ranging greatly in both length and difficulty. These attract climbers from all over the world who, unlike the early ascensionists, now have access to numerous climbing guidebooks, modern safety equipment, good information on climbing routes and technical difficulty, as well as weather forecasts and mountain accommodation and food. Glaciers A wide range of glaciers and glacial structures can be found across the Mont Blanc massif, all of which can be easily reached or viewed. Glaciers cover of the massif, of which fall within France. The Mer de Glace is the largest glacier in the western Alps, and the second largest in Europe. It has a total length of from highest snowfield to terminus and an area of 35–40 km2 (14–15 sq mi). At around in length, the Miage Glacier is Italy's longest glacier and also the largest debris-covered glacier in Europe. Other large glaciers include the Argentière Glacier (), the Saleina Glacier (), Trient Glacier (), the Bossons Glacier ( ) and the Brenva Glacier. Whilst these glaciers appear to show similar fluctuations in length, research shows that each glacier of the Mont Blanc massif has its own individual and distinctive response time to changes in snowfall and climate. The Bossons Glacier is known to respond first, then the Argentière and the Trient Glaciers respond four to seven years later, with the Mer de Glace reacting last —between eleven and fifteen years after changes are first observed in the movement of ice in the Bossons Glacier. Even the smallest glaciers can have a significant impact on the environment and on human activity. In 1892, a large body of water which had accumulated under the surface of the Tête Rousse Glacier, burst suddenly on the night of 11 July. It released of water and ice which flowed down the mountainside, killing 175 people in Saint-Gervais. The recent rediscovery of a further build-up of water deep within the glacier is now a cause of serious concern. The inexorable downward movement and melting of glaciers