How can you measure sea level?

Death valley is the lowest place in the US well over 100ft below sea level.

The Dead Sea that is located near or in Israel.The surface and shores of the Dead Sea are 422 metres (1,385 ft) below sea level.

if the land surrounding the sea is below sea level

Altitude

The Dead Sea is a salt lake, bordering Jordan to the east and Israel and the West Bank to the west. Its surface and shores are 423 metres below sea level.