A VTS operator is monitoring a deep-draft vessel and needs to ensure it stays clear of a known underwater obstruction. On the electronic chart display, the operator sees a symbol with the letters 'Wk' next to it. What does this symbol represent?

1. A designated anchorage spot for vessels with a specific draft.
2. A wreck, where any associated number indicates the cleared water depth above it at chart datum.
3. A wellhead or other underwater exploration device.
4. A warning point for a pilot boarding station.