Of the estimated 30,000 to 40,000 people who reached Dawson City during the gold rush, only around 15,000 to 20,000 finally became prospectors. Of these, no more than 4,000 struck gold and only a few hundred became rich. By the time most of the stampeders arrived in 1898, the best creeks had all been claimed, either by the long-term miners in the region, or by the first arrivals of the year before. The Bonanza, Eldorado, Hunker and Dominion Creeks were all taken, with almost 10,000 claims recorded by the authorities by July 1898; a new prospector would have to look further afield to find a claim of his own. Geologically, the region was permeated with veins of gold, forced to the surface by volcanic action and then worn away by the action of rivers and streams, leaving nuggets and gold dust in deposits known as placer gold. Some ores lay along the creek beds in lines of soil, typically 15 feet  to 30 feet  beneath the surface. Others, formed by even older streams, lay along the hilltops; these deposits were called "bench gold". Finding the gold was challenging. Initially, miners had assumed that all the gold would be along the existing creeks, and it was not until late in 1897 that the hilltops began to be mined. Gold was also unevenly distributed in the areas where it was found, which made prediction of good mining sites even more uncertain. The only way to be certain that gold was present was to conduct exploratory digging.. At a high estimate, how many more people who reached Dawson City during the gold rush actually became prospectors?