X-RAY REVERBERATION HAS EMERGED RECENTLY AS A POWERFUL TOOL TO PROBE THE ENVIRONS OF BLACK HOLES. X-RAY RADIATION EMITTED CLOSE TO THE BLACK HOLE IS SEEN DIRECTLY AND THROUGH REFLECTION OFF THE DENSE SURROUNDING MATTER. A TIME DELAY IS EXPECTED IN THE VARIABILITY OF THESE TWO COMPONENTS. CAREFUL ANALYSIS OF THESE LAGS THEIR CHANGE WITH ENERGY AND TIME-SCALE IS OFFERING A NEW TOOL TO LEARN ABOUT THE ENIGMATIC BLACK HOLE SURROUNDINGS. THIS PROPOSAL AIMS TO STUDY IRON K REVERBERATION IN MCG-5-23-16 AN OPTIMALLY BRIGHT AND VARIABLE AGN.USING DATA FROM OUR RECENTLY ACCEPTED PROPOSAL TO OBSERVE THE SOURCE FOR 300 KS WE WILL:1-MEASURE TIME-SCALE AND ENERGY-DEPENDENT TIME LAGS (USING A RECENTLY DEVELOPED MAXIMUM LIKELIHOOD TECHNIQUE). 2-STUDY THE DETAILED BROADBAND X-RAY SPECTRUM.THIS WILL ALLOW US TO CONSTRAIN THE EMISSION REGION BY DIRECTLY MEASURING LIGHT-CROSSING TIME DIFFERENCES. DETAILED LAG-ENERGY SPECTRA IN AT LEAST THREE TIME-SCALES WILL ALLOW US TO PROBE THE STRUCTURE OF THE INNER ACCRETION DISK IN AN UNPRECEDENTED MANNER THAT IS INDEPENDENT AND COMPLEMENTARY TO PURE SPECTROSCOPY.THE WORK IN THE PROJECT DIRECTLY ADDRESSES ONE OF THE QUESTIONS IN NASA SCIENCE GOALS IN ASTROPHYSICS: 'HOW DO MATTER ENERGY SPACE AND TIME BEHAVE UNDER THE EXTRAORDINARY CONDITIONS OF THE COSMOS?'. IN PARTICULAR THE PROJECT HAS THE OBJECTIVE UNDERSTANDING THE NATURE OF GRAVITY AT ITS EXTREME THROUGH BLACK HOLES AND THEIR ENVIRONMENTS.